Bring a large Pot of 2 tbsp olive oil to a boil. Cook the pasta according to package directions until al dente. Before draining, reserve 1 cup of pasta cooking water. Drain and set aside.

While the pasta cooks, heat 1 tablespoon of 2 tbsp olive oil in a large Skillet over medium-high heat. Add the 1 lb Italian sausage sweet or hot, casings removed, breaking it into bite-sized crumbles with a Wooden Spoon. Cook until deeply browned, about 6–8 minutes. Transfer the 1 lb Italian sausage sweet or hot, casings removed to a plate, leaving the drippings in the pan.

Add the remaining tablespoon of 2 tbsp olive oil to the same Skillet. Add the sliced 1 medium yellow onion halved and sliced into half-moons and 1 whole red bell pepper seeded and sliced into strips. Season with 1 tsp kosher salt plus more for pasta water and ½ tsp black pepper freshly ground. Sauté over medium-high heat, stirring occasionally, until the vegetables are softened and lightly caramelized, about 7–8 minutes.

Push the vegetables to the side of the pan and add the 4 cloves garlic minced, 2 tbsp tomato paste, 1 tsp smoked paprika, ½ tsp dried oregano, and ½ tsp red pepper flakes (if using). Stir and cook for 1–2 minutes until the 2 tbsp tomato paste darkens slightly and the 4 cloves garlic minced is fragrant.

Rinse peppers under cold water and pat dry. Slice off tops and bottoms, stand upright, and cut down along the inside to remove seeds and white ribs. Slice lengthwise into ½-inch strips; halve crosswise for shorter pieces. Keep sizes uniform so everything cooks evenly. If adding onion, slice it now. Measure out olive oil, salt, and pepper. Have everything ready before the pan goes on — sautéing moves fast.

Heat a large Skillet over medium heat. Add 1 tbsp olive oil per 2 peppers; warm 30–60 seconds until shimmering. Add pepper strips in a single layer — work in batches if crowded. Season with salt and pepper, stir to coat. Cook over medium heat, stirring every 2–3 minutes, for 6–10 minutes. At 6 minutes: slightly softened with snap. At 10 minutes: tender throughout with light color. For very soft peppers, cook 30–40 minutes over medium-low, stirring occasionally. Add garlic halfway through cooking. If browning too fast, add a splash of white wine or water and briefly cover to steam.