I've used commercial "steak" seitan (available in refrigerated cases in health food stores) added texture to what would have been a mushroom stroganoff and more surprisingly, a lot of flavor, too. I liked it a lot better than meat.
My homemade seitan was sliced 1/4 inch thick. I stir fried aromatics, then veggies, then seitan, finished with water mixed with either Marmite or Vegemite (really) to an iced tea color with a cup or so of water, followed by cornstarch dissolved in cold water to thicken the sauce. The result was something meaty but very light and yes, it fooled the occasional meat eater.
I never much liked seitan on its own as a sandwich filling. It needed veggies, a brown gravy, or mushrooms and sour cream.
The real thing is wheat flour with a little water, kneaded until the gluten is activated. Then it's kneaded under water to remove as much of the starch as one can, leaving the nearly fibrous gluten behind. I found it took about 3 changes of water in gallon glass jars. After the starch would settle, I'd pour off most of the water and dry the starch on cookie sheets in a low oven. I used whole wheat flour and what I got when the water was completely gone was extremely thin bran cracker, thinner than a potato chip. Crumbled, it was the best bran cereal I ever had. The gluten was turned into seitan by simmering it in a soy sauce based broth.
The restaurant variety is not true seitan, but it sounds like it would taste pretty good.