Put a large pot of salted water on to boil for the pasta.

Heat 1 tablespoon of the olive oil in a heavy skillet over medium-high heat. Add the sausage and sauté for 3 to 4 minutes or until lightly browned, breaking the meat up with a wooden spoon as it cooks. Transfer to a plate and drain the fat from the pan.

Return the pan to the stove over medium heat and add another tablespoon of olive oil. Add the onion and sauté until softened, 2 to 3 minutes. Add the garlic and cook until fragrant, 1 minute longer.

Add the red bell pepper and continue cooking, stirring often, until the peppers are crisp-tender, 2 minutes more. Add a pinch of crushed red pepper flakes and combine well.

Add a handful of spinach to the pan and toss with the onion mixture until wilted. Repeat the process until all the spinach is cooked, then return the crumbled sausage to the pan and combine well. Reduce the heat to low to keep warm.

Cook and drain the pasta according to package directions and toss with the remaining 2 tablespoons olive oil. Add the sausage mixture and combine well. Taste and adjust the seasoning as needed.

To serve, plate individual servings and top with grated cheese.