Mix 300 grams flour, 3 eggs, salt, and 130 ml water in a bowl to form a dough. Let rest for 5 minutes.
Preheat oven to 80°C (176°F).
Bring salted water to a boil in a saucepan. Use a spaetzle slicer to slice the dough into the boiling water. Cook until the spaetzle float to the surface, about 1-2 minutes, then remove with a slotted spoon. Drain and place in a buttered dish. Keep warm in the preheated 80°C (176°F) oven.
Cut all fillets into medallions of equal size. Heat 2 tablespoons oil in a pan over medium-high heat. Fry the medallions 2-3 minutes per side until browned. Season with salt and pepper. Wrap in aluminum foil and place in the preheated 80°C (176°F) oven.
In the same pan, fry the diced onions over medium heat for 5-7 minutes until golden brown. Add the diced garlic and cook for 1 minute. Deglaze the pan with 100 ml white wine. Add 150 ml meat stock and 50 g whipped cream. Bring to a simmer and cook for 5 minutes. Season with salt and pepper to taste. Remove the foil-wrapped fillets from the oven, unwrap them, and place in the sauce. Pour any meat juices from the foil into the sauce.
Remove the spaetzle from the oven. Increase oven temperature to 175°C (347°F).
Toss the spaetzle with some of the sauce and 1 tablespoon chives in the same dish. Arrange the meat medallions on top. Grate 150 g mountain cheese over the meat and pour the remaining sauce over everything. Place in the preheated 175°C (347°F) oven for 5-10 minutes until the cheese melts.