Sauté the onion and garlic.
Add tomato paste and cook for a few minutes.
Add crushed tomatoes, stock, and salt and pepper.
Add the gnocchi and spinach and mix to combine as best you can, submerging all the gnocchi.
Cover and bake, stirring occasionally, until the gnocchi are cooked and the spinach is wilted.
Add fresh mozzarella and broil until browned.