Dice the onion, mince the garlic, slice the bell peppers, and quarter the mushrooms.
Warm the olive oil in a large skillet or Dutch oven over medium heat. Add the onion and garlic to the pan, sautéing for 2-3 minutes until they become fragrant.
Add the bell peppers and mushrooms. Cook until softened and the mushrooms release their juices, about 5-7 minutes.
Stir in the crushed tomatoes, tomato paste, and vegetable broth. Add the oregano, basil, rosemary, and chili flakes if using. Season with salt and pepper.
Reduce the heat to low, cover, and simmer for 20-30 minutes, stirring occasionally, until the sauce thickens and the flavors meld.
If using olives, capers, or plant-based protein, stir them in during the last 5 minutes of cooking.
Pair the cacciatore with pasta, rice, crusty bread, or creamy polenta. Garnish with fresh basil if desired.