Bring a large pot of salted water to a boil. Add the penne pasta and cook according to package instructions until al dente. Drain and set aside.

In a skillet, heat olive oil over medium heat. Add the Italian sausage and cook until browned, breaking it up with a spoon, about 5-7 minutes.

Add chopped onion, garlic, and bell pepper to the skillet. Sauté until the vegetables are softened, about 5 minutes.

Stir in the diced tomatoes, spinach, oregano, basil, red pepper flakes, salt, and black pepper. Simmer for 10 minutes.

Add the cooked pasta to the skillet and toss to combine. Cook for an additional 2-3 minutes until heated through.

Serve hot, garnished with grated parmesan cheese.