To prepare meat: use a meat pounder to pound the meat to an even 1/2-inch thickness (I often use a rolling pin over plastic wrap). Cutting with the grain, slice the pounded meat into strips about 2 inches wide. Slice each piece of meat against the grain into 1/2-inch strips.
Pat beef dry with paper towels and season with salt and pepper. Heat 1 tablespoon oil in large skillet over medium-high heat until just smoking. Cook half of beef until well browned, 3 to 4 minutes per side. Transfer to medium bowl and repeat with 1 tablespoon more oil and remaining beef.
Heat remaining oil in now-empty skillet until shimmering. Cook mushrooms, onion, and 1/2 teaspoon salt until liquid from mushrooms has evaporated, about 8 minutes. (If pan becomes too brown, pour accumulated beef juices into skillet.) Stir in flour and cook for 30 seconds. Gradually stir in broths, then brandy and return beef and accumulated juices to pan. Bring to simmer, cover and cook over low heat until beef is tender, 30 to 35 minutes.
Stir noodles into beef mixture, cover and cook, stirring occasionally, until noodles are tender, 10 to 12 minutes. Off heat , stir in sour cream and lemon juice. Season with salt and pepper. Serve.