Drain the tomatoes. Wash the meat, dab dry and dice. Clean, wash and cut leek into rings. Heat oil in a pan. Fry the meat for 3-4 minutes until golden brown.
Take out the meat. Add the leek to the pan and fry for about 2 minutes, turning, seasoning with salt and pepper. Add 4-5 tablespoons of water and continue steaming for 3-4 minutes, turning.
In the meantime cut the tomatoes into strips. Let the dumplings simmer in boiling water for about 7 minutes. Remove the leek from the pan and let it drain. Wash the marjoram, dab dry and put something aside for garnishing.
Pluck off the remaining marjoram leaves and chop them coarsely. Drain the dumplings and let them drain. Melt the fat. Sweat flour in it. Add milk while stirring constantly, melt processed cheese in it and continue to cook for 2 minutes.
Stir in chopped marjoram. Season the sauce with salt, pepper and nutmeg. Arrange meat, leek, dumplings and tomatoes on 2 oven-proof trays or 1 suitable casserole dish. Pour cheese sauce over it and sprinkle with Gouda.
Bake in the preheated oven (electric cooker: 200 °C/ convection oven: 175 °C/ gas: level 3) for 10-15 minutes until golden brown. Remove the casserole and garnish with marjoram.