Brush the black salsifies vigorously under cold water. Then peel, wash and immediately place in vinegar water with 2 teaspoons of flour. Cook in salted water with lemon juice in a wide pot for about 15 minutes.
Clean, wash and chop the tomatoes. Peel onions and garlic. Chop both. Heat the oil. Sauté onion and garlic. Add the tomatoes and stew for about 5 minutes. Season with salt, pepper and sugar.
Melt the fat. Sweat 40 g flour in it. Deglaze with 1/2 litre water, milk and cream. Bring to the boil and stir in the stock. Let simmer for about 5 minutes while stirring. Season to taste with salt, pepper and nutmeg.
Lift out the black salsifies. Place half of the black salsifies close together in a greased casserole dish. Pour half the sauce over it. Cover with ham. Place the rest of the black salsifies on top. Layer the sauce, tomatoes and finally cheese.
Bake in the preheated oven (electric cooker: 200 °C/ convection oven: 175 °C/ gas: level 3) for approx. 30 minutes.