Onions peel and chop finely. Clean the mushrooms, possibly wash and halve them. Heat the oil in a pan and fry the mushrooms, turning them over. Add the onions and fry briefly.
Season with salt and pepper. Remove from the pan and set aside. Clean, wash and slice the tomatoes. Wash the sage, dab dry and pluck the leaves from the stalks. Grate cheese finely.
For the sauce, heat the fat in a saucepan, dust with flour, sauté and gradually deglaze with stock and cream, stirring constantly. Bring to the boil, stir in 75 g cheese and season with salt and pepper.
Stir in mushrooms and onions. Pour some sauce into a greased ovenproof dish, layer lasagne sheets, sage leaves (except for something to garnish), sauce and ham alternately in a dish.
Add the last layer of tomatoes. Spread the rest of the sauce and 25 g of cheese over it. Bake in the preheated oven (electric cooker: 175 °C/ convection oven: 150 °C/ gas: level 2) for about 50 minutes. If necessary, cover the lasagne after about 30 minutes of baking time.
Remove from the oven and serve garnished with sage.