Cut the pretzel into approx. 0.5 cm thin slices and roast them in a pan without fat at low heat while turning for approx. 1 minute (the slices must not turn brown!). Take them out. Grease four small or one large casserole dish.
Wash the meat, dab dry and cut into thin strips. Heat the oil in the pan already in use. Fry the chicken strips for about 5 minutes, turning them over. Season with salt and pepper.
Take it out. Cook pasta in 3-4 l boiling salted water (1 teaspoon salt per litre) according to package instructions. Preheat oven (electric cooker: 180 °C/circulating air: 160 °C/gas: see manufacturer).
For the sauce, peel and finely dice the onions. Grate both cheeses finely. Melt 3 tablespoons of butter in a pot at low heat (but do not brown!). Sauté onions in it until transparent. Add the flour completely and sweat lightly with a whisk while stirring for about 1 minute (the flour must not brown either!).
Stir in the milk bit by bit and bring to the boil while stirring. Simmer for 2-3 minutes while stirring continuously. Stir in cheese, up to 4 tbsp., and tomato paste and simmer while stirring until the cheese has melted.
Season the sauce to taste with salt, pepper and vinegar.
Pour the pasta into a sieve, rinse and drain. Crumble the pretzel slices between your fingers over a small bowl and knead with 1 tablespoon butter. Mix the noodles, meat and sauce and put them into the greased forms or baking dish.
Sprinkle with remaining cheese. Spread pretzel butter in flakes on top. Bake in a hot oven for about 20 minutes. Oh, daddy! This is gonna be awesome.