Cook the spaetzle in plenty of boiling, lightly salted water according to the instructions on the packet. Then drain and drain.
Cook the Brussels sprouts in boiling, also lightly salted water for about 8 minutes. Then drain, quench and drain.
Melt 2 tablespoons of butter in a saucepan, then stir in the flour and sauté briefly. Pour in the vegetable stock and milk while stirring, bring to the boil and then simmer over a low heat for about 5 minutes. Season to taste with a little salt and some freshly grated nutmeg.
Coarsely grate the mountain cheese. Put the spaetzle, cheese (except for something to sprinkle), Brussels sprouts and sauce in a buttered baking dish and mix together. Sprinkle with the rest of the cheese and bake in a preheated oven at 200 ° C (convection 175 ° C) for about 30 minutes.
In the meantime, peel the onion, cut into rings and turn in a little flour. Melt 1 tablespoon butter in a pan and roast the onion rings in it, seasoning with a little salt.
Rinse the chives, shake dry and cut into fine rolls.
Sprinkle the cheese spaetzle and Brussels sprouts bake with the fried onion rings and the chives and serve.