Place seitan in a large bowl. Slowly add 2 tablespoons of flour to bowl, stirring to coat all sides of seitan pieces. Sprinkle with ⅛ teaspoon of salt and pepper.
Heat 1 tablespoon of oil in a large skillet over medium heat. Add seitan and cook until browned on all sides, stirring often, about 3-5 minutes. Transfer to plate.
Return skillet to medium heat and add remaining tablespoon of oil. Add mushrooms, onion, rosemary, and remaining salt. Cook until onion is golden brown, about 5 minutes. Sprinkle vegetables with remaining flour and stir until coated. Stir in wine and cook for 1 minute. Add tomatoes, broth, roasted red peppers, and olives; reduce heat to medium low.
Once cacciatore comes to a simmer, add seitan. Continue to cook, stirring occasionally, until sauce has thickened, about 5 minutes. Serve over brown rice, couscous, or pasta and garnish with rosemary.