Cut the onion in half, then into thin slices. Heat the wine or water in a large skillet or pot and braise the onion until it is soft, about 5 minutes. Add the sliced mushrooms, bell peppers, garlic, and basil. Continue cooking over medium heat for 8 to 10 minutes, then stir in the dissolved miso, tomatoes, and pepper. Simmer 5 minutes.
Spread 1/2 cup of sauce evenly in the bottom of a large casserole dish. Arrange the seitan slices in the dish, then cover with the rest of the sauce. Cover the dish and bake for 20 to 25 minutes.