Melt the fat in a pot. Stir in flour and sweat briefly. Add milk and vegetable stock while stirring and bring to the boil. Let the sauce simmer for 5 minutes. Season to taste with salt, pepper and lemon juice.
Wash, clean and slice the zucchini. Wash the marjoram, dab dry and put something aside for garnishing. Pluck the remaining leaves from the stalks. Spread 4-5 tablespoons of sauce on the bottom of a large greased casserole dish, cover with lasagne plates.
Place zucchini slices close together on the lasagne plates, sprinkle with a little marjoram. Add 3-4 tablespoons of sauce and cover with boiled ham. Continue like this until all ingredients are used up.
Finish with slices of zucchini. Add the rest of the sauce and sprinkle with cheese. Bake lasagne in a preheated oven (electric cooker: 200 °C/ convection oven: 175 °C/ gas: level 3) for 35-40 minutes. Garnish with marjoram.