First of all, it's awesome that you're willing to cook veggie for your boyfriend. It usually really isn't any different than cooking with meat.... sometimes you just leave the meat out.
Other times, like with tuna casserole, it may take a little fussing to make a reasonable facsimile.
Is your boyfriend vegan or vegetarian? I'm going to assume vegan since that's what the title of this thread is.
There are some fishy "mock-meat" substitutes, usually available at Asian markets. If you're willing to go to that effort, maybe take your boyfriend with you to check labels...
Other than that, I'd suggest using a vegan product called tempeh
Tempeh is available at health food stores in the refridgerated section (or freezer) usually near the tofu.
Crumble up some tempeh and brown in a skillet (it'll probably stick and use more oil than you'd think). If you (or your boyfriend) want the "fishy" flavor, sprinkle 1 tsp. kelp powder (or any finely crumbled seaweed) over the tempeh. This will be your tuna substitute.
Here's the recipe I use for creamy noodle casseroles for my family. Just add the above "tuna substitute" to it:
12 oz uncooked pasta
12 oz. "tuna substitute"
1 medium stalk celery, minced
1/3 cup of diced onion
10 oz chopped fresh mushrooms (if you like mushrooms)
1 cup frozen peas
2 1/4 cup Silk Unsweetened Plain Soymilk
3 Tbsp white all-purpose flour
1 slice whole-wheat bread
salt nd pepper to taste
Set a pot of water on to boil. Cook the pasta in lightly salted water.
Preheat the oven to 350.
Lightly grease a 2qt casserole dish with a little oil.
Toast the bread and using a food processor turn into bread crumbs. (Or if you prefer crushed potato chips on top the casserole, skip the breadcrumbs)
Over med/low heat saute the onion, mushrooms & celery in a little oil, till done, using scant splashes of veggie broth to deglaze the pan as needed.
Pour 1+3/4 cups of the soymilk into the pan and bring to a simmer.
Combine the remaining milk w/the flour until completely dissolved. Slowly add to the pan stirring constantly. Simmer until the sauce has thickened, add frozen peas and then remove from heat.
In a large bowl combine the pasta, sauce & tuna substitute. Mix gently but well. Season w/salt & pepper to taste.
Place mixture into the casserole and sprinkle the bread crumb mixture (or crushed potato chips or whatever) over the top.
Bake for 35 minutes or until it's bubbly and the top begins to get golden brown & crusty.
Cool 5 minutes before serving.