Cook the pasta in plenty of boiling salted water for 8-10 minutes. Pour pasta onto a sieve and drain. Clean the spinach and wash it thoroughly. Peel and finely chop the onion. Heat the oil in a pot.
Fry the onion in it. Add spinach dripping wet and let it collapse. Season with salt and pepper. Clean, wash and slice the tomatoes. Whisk eggs and milk. Crumble the feta cheese into small pieces and stir into the egg milk.
Season with salt, pepper, oregano and nutmeg. Grease an ovenproof casserole dish. Mix the pasta, spinach and tomatoes and place in the casserole dish. Pour egg milk over it. Leave to set in the preheated oven (electric cooker: 200 °C/ gas: level 3) for approx. 20 minutes.
Grate the cheese and sprinkle it over the casserole 10 minutes before the end of the cooking time. Serve the pasta casserole hot.