Cook the pasta in plenty of boiling salted water for about 8 minutes. Drain, quench and drain well
Clean and wash the spinach. Peel and chop the onion and fry it in hot oil. Add the dripping wet spinach and let it collapse. Season. Drain on a sieve, possibly squeeze out a little more
Melt the fat. Sweat flour in it. Add milk and bring to the boil while stirring. Season to taste with salt, pepper and nutmeg. Take the sauce off the stove, stir in the egg yolk
Fill sauce, noodles, spinach and ham alternately in 2-3 layers into a large casserole dish (approx. 25 x 35 cm; 5 l capacity). Finish with sauce
Clean, wash and slice the tomatoes. Cover the lasagne with it. Grate cheese finely and sprinkle over it
Bake the lasagne in a preheated oven (electric cooker: 200 °C / convection oven: 175 °C / gas: level 3) for 35-40 minutes until golden brown