Step 1: Cook the pasta in a large pot of boiling salted water according to package directions until al dente. Reserve 1/2 cup of pasta water before draining.
Step 2: While the pasta cooks, heat olive oil in a large skillet over medium-high heat. Add the sausage and cook, breaking it into small pieces, until browned and cooked through (about 7-9 minutes).
Step 3: Add the diced onion to the skillet with the sausage. Sauté for 3-4 minutes until softened, then add the minced garlic and red pepper flakes, cooking for another minute until fragrant.
Step 4: Reduce the heat to medium-low and pour in the heavy cream. Let it simmer gently for 3-5 minutes until it begins to slightly thicken.
Step 5: Stir in the Parmesan cheese until melted and smooth. If using, add the spinach and stir until just wilted.
Step 6: Add the cooked pasta to the skillet and toss to coat thoroughly. If the sauce is too thick, add a bit of the reserved pasta water. Season with salt and pepper to taste and serve immediately.