Boil a large pot of water, add a generous pinch of salt, and cook pasta just to al dente according to package directions. Drain and set aside.
While pasta cooks, warm oil in a large skillet set over medium-high heat. Add sliced sausage and cook, stirring frequently, for about 5 minutes, until it begins to brown on the edges.
Reduce heat to medium-low. Add garlic to the pan and cook for 30-60 seconds, just until fragrant.
Add marinara sauce, cream or half and half, red pepper flakes, and salt and pepper to taste. Let the mixture simmer for 2-3 minutes, then turn off the heat. Stir in spinach. Cook 1 minute more, just until it wilts, then add cooked pasta and stir well.
Garnish with grated Parmesan, fresh parsley, and/or basil, if desired, and serve.