1) Brown the seitan in olive oil until somewhat crispy in a large pot.
2) Remove seitan from pot and sautee the onions and garlic on a low-medium heat for about 8 to 10 minutes.
3) Add the herbs, salt and pepper and stir. Add the flour and until it browns.
4) Add the broth and beer, whisking to remove lumps. Add the seitan and simmer on low heat for 2 hours, or medium-high heat for 30 minutes until the volume of liquid is reduced by half and the gravy takes on a silky, smooth, glossy texture and is a deep brown color.
Carbonnade a la flamande is a dish traditionally made in Belgium with beef, beef broth and the remaining ingredients. It's a stew that is traditionally simmered for 2 hours -- you can easily let the stew ingredients simmer on a low heat for that long, but if you're short on time, your goal is to reduce the volume of liquid by half - it'll still take on the silky texture and taste good, just won't be "traditional". I usually get this going first, and THEN prep for mashed potatoes and Brussels sprouts, so it cooks at least as long as it takes for the other two dishes to come together, which is also how it's traditionally served in Belgium. I've actually never had the real carbonnade a la flamande or carbonnade (which is made with red wine instead of beer), but my partner who used to eat meat once upon a time (and is Belgian) said it tasted and looked like the real thing, and that the texture was also spot on.
Source of recipe: I came up with a vegan version of a Belgian recipe, because there was (gasp!) some undrunk brown beer that lost its fizz!