Cook the pasta according to package directions and drain.
While the pasta cooks, start the sauce. Heat the oil in a large skillet over medium-high heat. Add the onion and cook for 5 minutes until they start to soften. Add the sausage and garlic. Cook the sausage for about 8 minutes, until it browns, breaking it into small pieces as it cooks.
Add the tomato sauce, basil, oregano, red pepper flakes, salt, and black pepper. Simmer for 5 minutes, stirring occasionally.
Turn the heat down to low and add the half and half, stirring to combine it with the tomato sauce. Cook for another 2-3 minutes. Add the spinach and stir until it wilts.
Add the drained pasta to the pan and gently mix it until it’s coated with the sauce.