Heat oven to 350° F. Bring a large pot of salted water to boiling. Coat a 13 x 9 x 2-inch baking dish with nonstick cooking spray.
Heat oil in a large skillet over medium-high heat. Season steak with 1/4 tsp each of the salt and pepper. Add steak to skillet in single layer. Cook 3 to 4 minutes, turning once. Remove to a plate.
Reduce heat to medium and add onion. Cook 3 minutes. Stir in mushrooms and sauté 5 minutes. Add remaining 1/4 tsp each salt and pepper. Sprinkle with flour. Cook 1 minute. Whisk in broth. Bring to a simmer and add sherry, if using. Simmer 3 minutes.
Cook noodles in boiling water for 5 minutes. Drain. Remove sauce from heat and slowly stir in meat and sour cream. Add noodles and spoon into prepared dish. Cover with foil; bake at 350° F for 20 minutes. Uncover and top with fried onions.