Stir eggs, milk and flour until smooth, season with salt. Allow to swell.
Peel and finely chop the onion. Heat butter in a pot and fry diced onion until transparent. Add spinach and 5 tbsp. water, bring to the boil and cook in a closed pot according to the instructions on the packet.
Season to taste with salt and nutmeg.
Clean the mushrooms and cut them in half. Heat 4 tablespoons of oil in a pan and fry the mushrooms for about 4 minutes. Season with salt. In the meantime heat 1 tsp. oil in a second pan (approx. 20 cm Ø) and pour in approx. 1/4 of the dough and fry on both sides until golden brown.
Fry 3 more pancakes in the same way.
Mix spinach and mushrooms. Coarsely grate the cheese. Fill pancakes with vegetables and sprinkle cheese. Bake in a preheated oven (electric cooker: 200 °C/ convection oven: 175 °C/ gas: see manufacturer) for approx. 10 minutes.