For the best flavor, heat the oil in a large skillet over medium-high heat. Add the onion and sauté until softened, about 5 minutes. Add the garlic, bell pepper, and mushrooms and cook for 2 minutes longer. Stir in the dried basil, fennel, and red pepper flakes. Alternatively, omit the oil and sauté these ingredients in a few tablespoons of water or combine them in a mic