Making homemade seitan for stew recipe, do I need to cook the seitan first, then cool it, then use it?
Raw seitan can work in these kind of dishes, but it needs a different kind of temperature regime than "just boiling" it. 

You want to avoid heating it through above the boiling point of water before it is well set, otherwise you will get a lot of expansion and a brain-y/fluffy texture that is far from meaty. Also, raw seitan pieces tend to recombine (it is glue protein after all). You will eventually need to heat it to boiling point to "finish" the texture.
The surface can be a bit "slimy" in the end result, probably due to remaining starch coagulating on it - this is not a problem in thick sauces but can be in thin and soupy dishes.

A strategy I found working well:


Cut raw seitan, so your thinnest dimension is a cm or less.
Sear surface with an open flame (brulee torch, stove burner...) to get some browning and to stop it from being sticky. Avoid overdoing it, you neither want strong expansion nor a strong burnt-hair taste. Do not saute instead, this will cause too much expansion.
Have your cooking liquid (broth,sauce,soup) ready, set up for a long simmer at a stable temperature. A rich or even emulsified liquid is preferrable, so add any fats (or things like coconut milk) now. Avoid adding strong acids or anything that could act as a protein tenderizer yet.  It should be well out of the food safety danger zone but safely BELOW ANY BOILING. 70-90°C. Repeat: You do not want it boiling even for a moment before the seitan is well set.
Add the seitan. Add anything else that benefits from long stewing. Make sure there is enough liquid to well cover it, some expansion can still happen. Leave alone to simmer for 3 hours.
Finish - bring to boil, and boil anything that needs boiling alongside, so you have a 15-30 min boiling phase. You will get some expansion but not overexpansion of the seitan pieces.