So I've been playing with seitan for months now on and off. At first I could only make rubbery, tough seitan (much like seitan o greatness) which was either only good in small cubes for stir fry, or thinly sliced for sandwiches.
I'm a formulations chemist, so I approached this like any other project. With time, resources and a scope of making soft, tender seitan; I have completed the project.
The Cliff notes:
1. Use vital wheat gluten
2. Add dry spices
3. Make a cup of your favorite flavoring broth
4. Add solid shorteing/fat to the dry powders and mix
5. Add liquid to dry mix and make a light dough
5. Simmer dough for 5-10 minutes in barely enough water
6. Serve as is or slice for stirfry.
Start with Vital wheat flour (from Whole foods, bulk section) which is ~70% protein content, skipping all the washing (more on why I prefer to skip all the washing later). Add dry spices directly to the flour and mix the dry ingredients without any liquids. Prepare a container of liquid flavors (soy sauce, aminos, etc) to be added to the dry portion.
I prefer to use vital wheat flour, because by eliminating the washing steps, you can put flavors in the powders and not wash them down the drain. Adding flavors to the powder also eliminates the long cooking time required to "reflavor" the seitan.
The following is the key part that took me the most time to find out. The kneading and cooking processes play a very small part compared to the ingredient composition. To control the texture, add a partially hydrogenated oil. You can use PH soybean oil if you like, but I use Crisco for availability. The oil choice will also change the amount of kneading needed to get your desired texture.
Add your oil of choice to the powder blend and using your hands, mash the dry ingredients and Crisco lump to your desired texture. The correlation between dry texture and cooked product is an experience thing. Add the flavored liquid portion and knead to desired consistency. Plop the whole thing in a pot and barely cover with broth, simmer for 5-10 minutes. Done.
I've found with crisco if you mash until the dry portion is very homogenous, the cooked material tends to be very tender, moist and uniform in texture. If you add enough crisco and knead for long enough, you can make a custard like substance that you can barely pickup with a fork. If you leave big chunks of crisco (and don't knead as much) the seitan has a little bit of grain to it, which works for "steaks".
Hope this helps a bit. The trick is the shortening. Try it and let me know what you think!