- Knead it very little
- Bake at a low temperature (325F for 90 minutes works for me)
- Add an ingredient that interferes with gluten formation (e.g. tomato paste)
I follow this recipe, with a few modifications.
I knead much less than recommended - just enough to bring the dough together, basically. The first time I made it I kneaded for a few minutes, and the seitan came out tough and chewy, just like you describe. Think about it: when we make bread, we want some chewiness, but flour is only 10% gluten so we have to knead a lot, and encourage most of the gluten to develop. Vital wheat gluten flour is 75% gluten, so we don't want all the gluten to develop or the seitan will be super rubbery, unless it's given a high water content by boiling.
I've also had seitan turn pretty rubbery (though not as bad as the over-kneaded stuff) when I accidentally set the oven too high. Even though I shortened the cooking time, the damage was done.
The recipe calls for tomato paste (and ketchup, but I add extra tomato paste instead). I've learned that changing the quantity of tomato paste dramatically affects the texture of the seitan. Skimping even a little will produce chewier seitan.
I've also tried adding an oniony flavor to seitan by caramelizing onions, pureeing them, and substituting the puree for some of the water in the seitan. To my surprise, the seitan turned out very soft and squishy, even though the wet:dry ratio was the same as ever. My theory is that tomato paste and pureed onions both contain something that interferes with gluten development. I've switched to using chopped pieces of caramelized onion rather than puree, since that doesn't interfere with the gluten as much, and it gives the seitan a nice sausage-like texture.