Heat the olive oil in a large, heavy-bottomed skillet or Dutch oven over medium-high heat. Add the whole sausage links and cook for 5-7 minutes, turning occasionally, until browned on all sides. The sausage does not need to be cooked through at this point. Remove the sausages from the skillet and set them aside on a plate.

Reduce the heat to medium. Add the sliced onions and bell peppers to the same skillet, using the rendered sausage fat for flavor. Sauté for 8-10 minutes, stirring occasionally, until the vegetables are soft and the onions are translucent. Scrape up any browned bits from the bottom of the pan.

Add the minced garlic, dried oregano, dried basil, and optional red pepper flakes to the skillet. Cook for another minute until the garlic is fragrant, stirring constantly to prevent it from burning.

While the vegetables are cooking, slice the browned sausage into 1-inch thick rounds on a bias.

Return the sliced sausage to the skillet with the vegetables. Pour in the crushed tomatoes, and season with salt and black pepper. Stir everything together to combine.

Bring the mixture to a gentle simmer. Once simmering, reduce the heat to low, cover the skillet, and let it cook for 15-20 minutes. This allows the sausage to finish cooking and the flavors to meld together. The sauce should thicken slightly.

Remove the lid and taste the sauce. Adjust seasoning with more salt, pepper, or red pepper flakes if needed. Stir in the fresh parsley or basil just before serving.

Serve hot on its own, with crusty bread, over pasta, polenta, or piled into toasted hoagie rolls for classic sausage and pepper sandwiches.