Peel the asparagus, cut off the lower woody ends. Put the asparagus in plenty of boiling salted water, add some sugar and a teaspoon (10 g) of butter and cook for 15-20 minutes, depending on the thickness.
In the meantime, clean and slice the mushrooms. Peel onion and chop very finely. Wash and clean the tomato. Quarter the tomato, remove seeds and cut the flesh into fine cubes.
Finely grate the cheese. Drain asparagus well. Measure 1/8 litre of asparagus water. Melt the rest of the butter in a pan and fry the onion until transparent. Add the mushrooms and fry while turning. Dust with flour and sauté briefly.
Add milk and the asparagus water while stirring, bring to the boil and simmer for about 5 minutes. Melt half of the cheese in the sauce. Season to taste with salt and pepper. Roast the toast bread in a toaster.
Place on a baking tray lined with baking paper. Place two slices of ham and a quarter of the asparagus on each slice of toast. Spread the sauce over the toasts and sprinkle with remaining cheese and tomato cubes.
Bake the toast in a preheated oven (electric cooker: 200 °C/ convection oven: 175 °C/ gas: level 3) for 10-15 minutes. Wash the chives, dab dry and cut into small rolls. Serve toast sprinkled with chives.