Sauté briefly onion and garlic in hot oil until soft. Add ground beef, stir fry about 5 minutes over medium heat.
Add chopped tomatoes, stir, add 1/2 cup water, bring to the boil. Reduce flame, add oregano or basil, salt and pepper. Simmer gently for about 20 minutes.
Tip half the spinach into a colander and pour over boiling water to wilt, then repeat with the second half. Squeeze out as much moisture from the spinach as you can, then leave to drain in the colander.
Cook Schupfnudeln or gnocchi as usual, drain. Stir the Schupfnudeln or gnocchi and spinach into the Bolognese, then tip into a large ovenproof dish.
Tear the mozzarella into chunks, scatter over, bake at 220°C until golden and bubbling.