Bring a large pot of salted water to a boil. Wash and dry all produce. Halve, peel, and thinly slice onion. Core, deseed, and thinly slice bell pepper. Mince parsley leaves and stems. Remove sausage from casings; discard casings.

Heat a large drizzle of oil in a large pan over medium-high heat. Add onion, bell pepper, and half the Tuscan Heat Spice. Cook, stirring, until slightly softened, 3-4 minutes.

Add sausage and another drizzle of oil to pan. Cook, breaking up meat into pieces, until browned and cooked through, 5-7 minutes.

Once water is boiling, add spaghetti to pot. Cook, stirring occasionally, until al dente, 9-11 minutes. Reserve 1 cup pasta cooking water (1½ cups for 4 servings), then drain.

Once sausage is cooked through, add diced tomatoes to pan. Season with salt, pepper, and remaining Tuscan Heat Spice. Bring to a boil, then lower heat and let simmer while spaghetti cooks. After you’ve drained spaghetti, stir stock concentrate and ¾ cup reserved pasta cooking water (1 cup for 4 servings) into pan. Continue simmering until slightly reduced, 1-2 minutes more.

Add spaghetti to pan; toss to coat. (For 4 servings, carefully transfer everything into pot used for pasta.) Stir in 1 TBSP butter (2 TBSP for 4), half the parsley, and half the Parmesan. (TIP: If needed, stir in remaining reserved pasta cooking water a splash at a time until spaghetti is thoroughly coated in sauce.) Season with salt and pepper. Divide between bowls and sprinkle with remaining parsley and Parmesan.