In a large, deep skillet over medium heat, cook sausage, stirring frequently, until browned. Use a slotted spoon to transfer sausage to a plate lined with paper towels.
Add onion to the fat in the skillet and sauté until softened, 6-8 minutes. Add garlic, oregano, and pepper flakes and cook 1 minute more.
Add stock or water, tomatoes, and spaghetti to skillet. Bring to a boil and cook, stirring occasionally, for 5 minutes.
Add cauliflower and cook until pasta is tender, 3-5 minutes more. If liquid evaporates before pasta is done cooking, add more and continue to cook.
When pasta is tender, take skillet off heat and stir in cooked sausage, basil, and Parmesan. Serve immediately.