Heat butter and olive oil in a large nonstick skillet over low heat. Add onion and cook until very soft (light-to-medium brown), slowly.
Add garlic to pan; cook about 2 minutes. Add preserved lemon paste and stir, mixing it into the onions and garlic.
Increase heat to medium. Add gnocchi and spinach to pan; cook until spinach wilts, stirring constantly. Add pepper to taste, then sprinkle with cheese. Pecorino is saltier than parmesan, so you probably won't need to add salt, but taste and see.