Step 1: Heat olive oil in a large, heavy-bottomed skillet or Dutch oven over medium-high heat. Add the Italian sausage links and brown them on all sides until they have a golden crust. This should take about 8-10 minutes. Remove the sausage from the skillet and set aside. Leave about 1-2 tablespoons of rendered fat in the pan.
Step 2: Add the sliced onions to the skillet. Cook over medium heat, stirring occasionally, until softened and translucent, about 5-7 minutes.
Step 3: Add the sliced bell peppers to the skillet with the onions. Continue to cook, stirring occasionally, for another 8-10 minutes, until the peppers are tender-crisp. Add the minced garlic during the last minute of cooking the vegetables and stir until fragrant.
Step 4: Pour in the crushed tomatoes and stir in the tomato paste, dried oregano, dried basil, and red pepper flakes (if using). Season with salt and freshly ground black pepper. Stir well to combine all ingredients.
Step 5: Return the browned sausage links to the skillet, nestling them among the vegetables and sauce. Bring the sauce to a gentle simmer, then reduce the heat to low, cover the skillet, and let it cook for at least 25-30 minutes, or until the sausage is cooked through and tender, and the flavors have melded. Stir occasionally to prevent sticking.
Step 6: Taste the sauce and adjust seasonings if necessary. Garnish with fresh chopped parsley before serving.