Cook the tagliatelle pasta until al dente according to the instructions on the packet.
Cut the meat into even strips. Clean the mushrooms and cut into even pieces. Finely chop the onion and mash the peppercorns with the knife.
Heat the oil in a frying pan. Season the meat with salt and pepper and fry quickly in the oil. Then lift out of the pan and keep warm. Sweat the onions in the frying fat, add the mushrooms and peppercorns, deglaze with beer and reduce a little. Season with salt and pour in the beef soup. Mix the sour cream with the flour and mustard until smooth and stir into the sauce. Let it simmer until creamy.
Put the meat back in, bring the sauce to the boil again briefly, season with salt, pepper and lemon juice and serve with the pasta.