Remove the membrane from the sweetbreads. Cover with some salted boiling water and soak for a few minutes.
Finely slice the celeriac, the carrots and the parsley and poach them for 15 minutes in the stock and the milk. Add the sweetbreads to the mixture and steep for 10 minutes on low heat.
Slice the sweetbreads into 2 cm slices.
Finely slice the raw ham and the onions, sauté in butter, stir in the flour and then add the stock and the milk. Let cook for about 5 minutes while stirring.
Strain the mixture, mix in the cheese, season with salt and pepper and then stir on high heat until you obtain a creamy sauce.
Butter the four slices of bread and toast them under the grill.
Place a slice of cooked ham on each slice of bread and arrange the sweetbreads on top of the ham. Spoon over the cheese sauce, place the slices of bread in an oven dish, grill until golden and serve with the finely sliced Chinese cabbage.