Wash the potatoes and boil in water for about 20 minutes. Wash spinach and drain very well. Peel and halve the onion and cut into fine cubes. Leave the diced bacon out in a pan or wide pot until the bacon is slightly crisp.
Drain the bacon cubes on kitchen paper. Fry the onion cubes in the bacon fat for 1-2 minutes, then add the spinach. Steam spinach for 2-3 minutes and season with salt, pepper and nutmeg. Put the spinach on a plate.
Wash, clean, quarter and seed the tomatoes. Cut the flesh into small cubes. Melt the fat in a saucepan. Stir in flour and sweat for 1-2 minutes. Add the milk while stirring. Then add processed cheese.
Simmer the sauce while stirring for about 1 minute. Season sauce with salt, pepper and nutmeg. Drain potatoes, rinse with cold water and peel. Divide the potatoes and spinach into 4 portion moulds.
Add tomato cubes and sauce and sprinkle with cheese. Bake the potatoes in a preheated oven (electric cooker: 200 °C/ convection oven: 175 °C/ gas: level 3) for 5-10 minutes until golden brown.