Wash and clean the spinach. Boil up salted water and blanch the spinach in portions. Quench, squeeze out the water well. Chop the spinach roughly. Melt the fat in a pot. Add flour and let it sweat for a short time.
Add milk and stock while stirring constantly and bring to the boil. Season with salt, pepper and nutmeg and set aside. Peel onion and garlic. Dice onion and press garlic through a garlic press.
Heat the oil in a pan. Sauté the onion and garlic until transparent. Add the spinach and cook for about 3 minutes. Season to taste with salt and pepper. Wash, clean and slice the tomatoes.
Cut the ham into strips. Grease an oven dish and spread the bottom with some sauce. Layer the lasagne sheets, spinach, tomatoes, ham and sauce in order. Repeat until all ingredients are used up.
Bake in the preheated oven (electric: 175 °C/ gas: level 2) for about 20 minutes. Sprinkle with cheese and bake for another 10 minutes. Serve warm and garnish with basil as desired.