Stem, seed, and slice both green and red bell peppers into thin ¼" strips. Peel and halve onion. Slice halves thinly. Mince garlic. Remove sausage from casing.

Heat 1 tsp. olive oil in a large pan over medium-high heat. Add Italian sausage links and cook until browned on two sides, 4-5 minutes per side. Remove sausages from pan, let cool 5 minutes, and, on a separate cutting board, slice into ½" pieces on an angle. Sausages will finish cooking in a later step. Wipe pan clean and reserve. While sausages cook, cook pasta.

Add pasta to boiling water. Cook until al dente, 11-13 minutes. Reserve ½ cup pasta cooking water, then drain pasta in colander. Return pasta to pot and toss with 2 tsp. olive oil to prevent sticking. Set aside. While pasta boils, cook vegetables.

Return pan used to cook sausages to medium-high heat and add 1 tsp. olive oil. Add peppers, onion, garlic, and red pepper flakes (to taste) to hot pan. Reduce heat to medium, stir, and cover. Cook until vegetables are softened, 5-7 minutes.

Add sausage pieces, marinara, and reserved pasta cooking water to pan. Simmer, stirring occasionally, 4-5 minutes. Stir in spinach, pasta, half the Parmesan (reserve remaining for garnish). Cook until spinach wilts slightly and sausages reach a minimum internal temperature of 165 degrees, 1-2 minutes. Place pasta on a plate or in a shallow bowl. Garnish with Parmesan.