Bring a large pot of salted water to a boil. Wash and dry produce.

Halve, peel, and thinly slice onion. Halve, core, and thinly slice bell pepper into strips. Mince parsley.

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

Remove sausage* from casing if necessary; discard casing. Add sausage and another drizzle of oil to pan. Cook, breaking up meat into pieces, until browned and cooked through, 4-6 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.

While spaghetti cooks, add crushed tomatoes to pan with cooked sausage. Season with remaining Tuscan Heat Spice, salt, and pepper. Bring to a boil, then reduce to a simmer and cook until thickened, 4-6 minutes.

Stir in stock concentrate and ¾ cup reserved pasta cooking water (1 cup for 4 servings). Cook until slightly reduced, 1-2 minutes more.

Add spaghetti to pan with sauce; 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 more reserved pasta cooking water a splash at a time until spaghetti is thoroughly coated in sauce.) Season with salt and pepper.

Divide pasta between bowls. Sprinkle with remaining parsley and remaining Parmesan and serve.