Cook pasta according to package directions, setting aside 1-2 cups of pasta water before draining. Rinse with cold water and set aside.
Small dice yellow onion. Heat a Dutch oven with 1 tbsp vegetable oil. Add onion and some salt and cook, stirring occasionally, until onion is soft and golden.
Mince garlic and add to onion once onion is golden brown. Cook for 1-2 minutes, until fragrant.
Slice sausages into rounds. Cook in a nonstick skillet over medium heat until browned. Add sausages to Dutch oven with onion. Add red pepper flakes, salt, and pepper. Add pasta. Grate Parmesan and add along with the spinach. Stir until spinach wilts.
Serve with extra Parmesan as desired.