In a small bowl combine the mustard, 1-1/2 teaspoon sugar, a pinch of salt and 1 tablespoon water to form a thick paste. Let the mustard paste mixture rest for 15 minutes.
Heat 2 tablespoons of the oil in a large skillet on high heat, add the mushrooms and the onions, cover the pan and reduce the heat to low. Stirring occasionally, simmer 20-30 minutes, or until the vegetables are soft. Drain in sieve and return to skillet, discard the liquid. Set aside.
Lay each slice of beef and slice into ¼” wide strips. Heat 2 tablespoons of oil in a large skillet, drop in half the meat and fry for 2 minutes stirring constantly, until meat is light brown. Add the finished meat to the vegetables in other skillet and repeat with remaining meat. Adding additional oil if needed. After all meat is combined with vegetables add remaining salt and pepper, and the mustard paste. Stir in the sour cream, a tablespoon at a time, then add remaining ½ teaspoon of sugar and reduce the heat to low. Cover the pan and simmer 2-3 minutes, or until sauce is heated through.
Cook egg noodles according to package directions.
Transfer meat to a heated serving platter and serve with cooked egg noodles.