Cook noodles in a 6- to 8-quart pot of boiling salted water until just tender. Drain in a colander. Meanwhile, thinly slice onion using slicer. Heat oil in a 12-inch heavy skillet over high heat until hot but not smoking. While oil heats, pat steaks dry. Sprinkle 3/4 teaspoon salt and 1/4 teaspoon pepper on steaks, then add to skillet and saute, turning over once with tongs, about 6 minutes total for medium-rare. Transfer to a plate with tongs. Add onion, garlic, and remaining 1/2 teaspoon salt and 1/2 teaspoon pepper to oil in skillet and saute, scraping up any brown bits with a wooden spoon, until onion begins to brown, about 3 minutes. Add mushrooms and saute, stirring occasionally, until liquid given off by mushrooms is evaporated, about 4 minutes. Add brandy and boil until evaporated, about 1 minute. Add broth and Worcestershire sauce, then cover and cook until onion is softened, about 4 minutes. Remove from heat and stir in sour cream. Toss noodles with some of sauce and serve remaining sauce over steaks.