Preheat a large, lidded skillet over medium-high heat. Saute the seitan for about 2 minutes. Add the onions and saute for another 5 minutes, until softened.
Add the mushrooms, garlic, thyme, basil, salt, and pepper, and saute for another 8 minutes. Once the mushrooms are cooked and soft, add the wine and broth. Add the spinach in batches and use tongs to incorporate them with everything else. Cook for about 5 more minutes.
Serve immediately with slices of lemon, if desired.