When I first went vegan I was already mildly underweight and I dropped more weight very quickly so I had to work to put it back on. It is fairly common for new vegans to initially lose weight as they figure out what to eat and how to eat on a regular basis on a plant based diet. I see in your previous posts that you are struggling to figure out what to eat. For a short time I had this problem too. I poured over recipes on blogs and vegan websites, checked out cookbooks from libraries and bookstores, and went to work experimenting. I loved to cook before so it was an awesome and fun challenge for me. But at first, because I went vegan overnight from omnivore, I just ate what I was already familiar with that was naturally vegan...lentils, oats, almond milk (already had intolerance to dairy so barely ate that before anyway), fruits and vegetables. My repertoire expanded tremendously over time but it takes some patience and willingness to learn and experiment. I found that I used a lot more beans and whole grains than I previously did as an omnivore so I had to figure out just what to do with them. It didn't take too long lol. I expanded my grains beyond the typical brown rice and oats to include wild rices, millet, buckwheat groats, quinoa, couscous, bulgur wheat etc. I found that bulgur has a "meaty" ground beef like texture and look and goes well with tacos or in chili or spaghetti sauce with red lentils for an extra protein boost and flavor. I would add tomatoes, spices and seasonings, and other vegetables etc to it too. It goes well as a sloppy joe sauce in sandwiches too. Buckwheat groats are great as a raw soaked cereal to go with fresh fruit and almonds for a nice crunchy breakfast. Millet makes a nice base for red lentil dahl, with Aloo Gobi, or with tropical fruits and chickpeas mixed in it for breakfast etc. Wild rice goes well in a lentil cabbage rice soup.
Some foods that helped me gain weight...avocados. I made avocado basil pesto a lot and added it to grain and vegetable/bean dishes or in sandwiches with tomatoes and a heavy seeded bread. it can also be used as a salad dressing. Another one is to blend avocado with banana and cocoa powder and a touch of plant milk or water. Add a little maple syrup or other sweetener if you wish. makes a great pudding or even frosting if you make it thick enough. I snacked on medjool dates. They are like dessert. One has over 40 calories on average and it is easy to eat a fair bit of them. I added frozen bananas to a lot of smoothies, and did use vegan protein powder in the beginning but do not rely on it too much anymore. Banana and a nut or peanut butter combined makes a good breakfast, especially with oatmeal. Couscous is another quick cooking grain you can throw in fresh or frozen fruit and nuts into for breakfast. I have bought nuts from the baking section of grocery stores that were less expensive, though maybe not as high quality. Seeds tend to be cheaper. I would snack on whole almonds and I still use almonds to make my own homemade almond based mayonnaise (I sometimes use the skinned and blanched almonds found in baking sections of groceries to do this). I even learned to make homemade almond "cheese" slices. If you can find it (usually at natural or whole foods stores or in foreign ethnic sections of grocery stores) tahini makes a nice ingredient that goes in hummus, or soups, or in sauces that you can pour over dishes. I make a "cheese" sauce by mixing nutritional yeast, tahini, almond milk, pumpkin or sweet potato, dijon, and various spices in a blender and then heat it on the stove. I use it for macaroni and "cheese" or over baked potatoes with steamed broccoli.
I often made my own homemade breads, but when I would buy them I would buy seeded heavier breads with a lot of nutrition compared to the standard industrial bread. Ezekiel bread (can be found in frozen section of groceries) is a decent vegan commercial bread, as is Rudys. They are expensive though, hence why I made my own bread. After making a dozen batches it becomes second nature and part of your routine and is no big deal. I make several loaves at a time and freeze them for future use. I would add homemade white bean dip or hummus to them for sandwiches. Bean dips can be made in mere minutes with a blender. I used to wonder what kinds of sandwiches vegans eat lol. I learned to slice tempeh and add it to a sandwich and add saurkraut and a homemade tahini dressing or mayo and my was that filling and good! Tempeh is fairly high protein and nutritious. I used to only find it at Whole Foods stores but learned that I could and did make requests for mainstream stores to include tempeh and other items to their shelves and they always have so now I can find tempeh at my regular supermarket. Most larger chain groceries have online forms or you can go to their customer service counter and fill out a form requesting a specific food to be added to their store. They will usually contact you as to whether they can or not. Also, bean burgers or vegetable/grain burgers are great too.
Fruits like pears, mango, banana, pineapple are all more calorie dense and I use them for snacks at work or with breakfasts. I can cut up a pineapple and store the cut pieces in a tight fitting tupperware container and it keeps a whole week and I just take out what I need each day. I have made my own energy bars, like a ton of them, and then freeze them until I need them for snacks or when out backpacking or camping etc. I like the following site to figure out how to make my own bars:
I could go on and on with the variety of food you can eat as a vegan, and healthy too, to gain and maintain weight that isn't remotely expensive. No need to buy designer bars or protein drinks or commercial fake cheeses or meats etc if you are trying to save money. I am on a very tight budget now having finished school and owing student loans etc while only working part time right now. I am also in recovery from an eating disorder and finally got to a normal healthy weight range and I am trying to maintain that. Even so, I am not bored with my choices. I started making my own plant milks in lieu of buying commercial ones that tend to be more expensive (soy milk is the cheapest but due to thyroid issues I have to keep my intake to a moderate level). I can buy a lb of flax seeds and make five cups of plant milk with a 1/4 cup of seeds and water and a little sweetener (a few fresh dates). I started doing this a month ago and still haven't gone through all the flax seeds I bought or had to buy any more dates since it only takes a few to sweeten a whole batch and I have probably saved $12 or more. I do add calcium powder and vitamin D drops to my homemade milks but again I buy one container of each of those every three or four months for relatively cheap through amazon.com so no big deal. And I get a daily dose of omega 3s with the flax milk. You can make your own plant milk with rice, oats, cashews, almonds, sesame seeds, you name it. Some ingredients will be more expensive than others.
At any rate I better close up here. Vegweb was a great help for me in the beginning as they have a HUGE recipe section. Hope this helps!