Defrost the spinach. Dab the fillet dry and cut into thin slices. Heat milk. Grate fontina, crumble gorgonzola. Dissolve both cheeses (up to approx. 2 tablespoons Fontina) in the milk while stirring.
Heat 1 tablespoon of oil in a frying pan, fry the gnocchi in it for about 2 minutes, turning them until golden brown and take them out. Heat 1 tbsp. oil in a frying pan again. Fry the meat over a high heat while turning. Season with salt and pepper.
Preheat oven (electric cooker: 200 °C/circulating air: 180 °C/gas: see manufacturer). Squeeze the spinach well and put it in an ovenproof dish (approx. 15 x 20 cm), season with salt, pepper and nutmeg. Spread gnocchi and pork tenderloin on top. Pour cheese sauce over it. Sprinkle with rest of fontina. Bake in a hot oven for about 20 minutes until golden brown.