Bring a good 3 litres of salted water (1 teaspoon salt per litre) to the boil. Peel and finely dice the onions. Cut the cucumbers into slices. Cook spaetzle in boiling salted water according to package instructions.
Dab steaks dry, cut them crosswise to the meat fibre in strips of approx. 1 cm width. If necessary, cut them in half lengthwise beforehand. Heat up the lard in a pan.
Add just enough strips of meat so that they lie next to each other and fry for 2-3 minutes, turning. Season with salt and pepper. Fry the rest of the meat in the same way. Take out.
Fry the diced onions in the frying fat until transparent. Dust with flour and sauté lightly for 2-3 minutes while stirring. Add stock and cream while stirring and simmer for 3-4 minutes.
Drain the spaetzle. Preheat oven (electric cooker: 200°C/circulating air: 175°C/gas: level 3). Season the sauce with salt, pepper and mustard. Wash parsley, shake dry and chop.
Add meat, cucumber and parsley to the sauce. Put them into four small or one large casserole dish and spread the spaetzle on top. Grate cheese over it. Bake in a hot oven for 18-20 minutes.