Heat avocado oil in a large, nonstick skillet over medium-high heat. Add onion and cook until softened, 3 to 4 minutes. Add garlic and cook just until fragrant, about 30 seconds. Add sausage; stir to break up the meat and cook until well browned, 5 to 7 minutes.
Drain excess oil and keep meat mixture in the skillet. Add gnocchi, stirring to break up any clumps. Stir in tomatoes, salt, basil, oregano, and pepper. Cover and simmer for 5 minutes.
Stir in spinach and nutmeg and simmer until spinach has wilted, 3 to 5 minutes. Sprinkle mozzarella and Parmesan cheeses on top, cover, and simmer until cheese has melted, 2 to 3 minutes. Serve immediately.
