Brush the black salsifies vigorously under running water, clean, peel and wash again. Immediately place them in weak vinegar water so that they do not discolour. Cook in boiling salt water for about 20 minutes.
Clean and wash the leek, cut lengthwise and carefully remove the long leaves. Leave to stand in boiling salted water for about 2 minutes. Cut the ham into approx. 4 cm wide strips. Heat the fat in a pot.
Sweat flour in it until golden brown. Deglaze with milk and 1/4 litre of the vegetable water and bring to the boil. Season to taste with salt and pepper. Add 150 g cheese and let it melt while stirring. Put the sauce aside.
Drain black salsify, let it cool a little. Grease the baking dish. Wrap the black salsifies in ham and leek. Pour the sauce over it and sprinkle with the rest of the cheese.
Bake in the preheated oven (electric oven: 175°C/ gas: level 2) for 30-40 minutes until golden brown. Remove from the oven, garnish with parsley and serve immediately.