Prepare spaetzle in boiling salted water according to package instructions, drain. Then rinse with cold water and drain. Meanwhile wash the meat, dab dry and cut into strips.
Peel and finely dice the onions. Halve cucumbers lengthwise and cut into slices. Heat oil in a large, coated pan. Sauté the meat in it over a high heat while turning for about 2 minutes.
Add onions and cucumbers and continue frying for 1-2 minutes. Dust everything with flour, sweat briefly and deglaze with stock, milk and cream. Let simmer for 2-3 minutes. Stir in mustard and season to taste with salt and pepper.
Wash parsley, dab dry and pluck leaves from the stalks. Set some leaves aside for garnishing, cut the remaining leaves into strips. Fill the shredded leaves into a suitable gratin dish.
Mix the spaetzle with the parsley and add to the shredded meat. Grate the cheese and spread on top. Bake in the preheated oven (electric cooker: 200 °C/ convection oven: 175 °C/ gas: level 3) for 10-15 minutes.
Remove and garnish with parsley.