Wash and dry all produce. Bring a large pot of salted water to a boil. Halve, peel, and thinly slice onion from pole to pole into strands. Core and seed bell pepper, then cut into thin strips. Mince parsley. Place tomatoes in a medium bowl and break up into small pieces with your hands (careful, they may splatter).

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, tossing, until slightly softened, 3-4 minutes.

Remove sausage from casings, then add to pan with veggies along with a drizzle of oil, breaking up meat into pieces. Cook, tossing occasionally, until sausage is cooked through and browned at edges, about 5 minutes.

Once water boils, add spaghetti to pot. Cook, stirring occasionally, until al dente, 9-11 minutes. Carefully scoop out and reserve 1 cup pasta cooking water, then drain.

Meanwhile, once sausage is cooked through, add tomatoes to pan. Season with salt, pepper, and remaining Tuscan heat spice, then give everything a stir. Bring to a boil, then lower heat and let simmer while pasta cooks. After you’ve drained spaghetti, stir ¾ cup pasta cooking water and stock concentrate into pan. Continue simmering until slightly reduced, 1-2 minutes.

Add spaghetti to pan, tossing to coat. (TIP: Add more pasta cooking water if needed to give sauce a loose consistency.) Stir in 1 TBSP butter, half the parsley, and half the Parmesan. Season with salt and pepper. Divide between bowls, then sprinkle with remaining parsley and Parmesan.