In a large pot of boiling salted water, cook the penne pasta according to package instructions until al dente. Drain and set aside.
In a large skillet over medium heat, heat the olive oil. Add the spicy Italian sausage and cook, breaking it apart with a spatula, until browned and cooked through (about 5-7 minutes). Drain excess fat if necessary.
Add the chopped onion to the skillet and sauté until translucent (about 3-4 minutes). Stir in the minced garlic and cook for an additional minute until fragrant.
Pour in the crushed tomatoes and add the red pepper flakes, dried oregano, salt, and pepper. Stir to combine and let the mixture simmer for about 10 minutes, allowing the flavors to meld.
Stir in the fresh spinach and cooked pasta until everything is well coated in the sauce and the spinach is wilted (about 2-3 minutes). Adjust seasoning with salt and pepper if needed.
Serve the pasta in bowls, topped with a generous sprinkle of grated Parmesan cheese and garnished with fresh basil leaves.
Notes
For an elegant finish, drizzle a bit of extra virgin olive oil over the plated pasta and sprinkle some extra red pepper flakes for added color and spice.