In a 5- to 6-quart pan over high heat, bring 3 to 4 quarts water to a boil. Add noodles, return to a boil, and cook, stirring occasionally, until barely tender to bite, 5 to 7 minutes. Drain and pour into a large serving bowl.
Meanwhile, rinse beef and pat dry. Cut meat across the grain into 1/8-inch-thick slices.
Pour 1 tablespoon olive oil into a 12-inch frying pan or 14-inch wok over medium-high heat; when hot, add beef and stir until browned on the edges but still pink in the center (cut to test), 2 to 3 minutes. Pour into another bowl.
Add remaining 2 tablespoons olive oil to pan; when hot, add onion and garlic and stir until onion is limp, 3 to 5 minutes. Add mushrooms and stir often until beginning to brown, 3 to 5 minutes.
Add wine, cognac, and oyster sauce to mushroom mixture. Bring to a boil over high heat, then reduce heat and simmer to blend flavors, about 5 minutes.
Add cream and return to a simmer. In a small bowl, mix cornstarch with 1 tablespoon cold water until smooth. Add to mushroom mixture and stir until it boils and thickens, about 2 minutes.
Add sour cream and beef with any accumulated juices to pan and stir just until heated through, about 1 minute. Add salt and pepper to taste.
Pour beef Stroganoff over hot noodles. Sprinkle with parsley if desired.
Cleaning mushrooms: Trim off tough or discolored bottoms of mushroom stems and any bruised spots or blemishes. (For shiitakes and oysters, remove the entire fibrous stem.) For firm mushrooms such as portabellas, wipe dirt off with a damp cloth or place in a colander, rinse thoroughly under cool running water, and pat dry with towels. For delicate mushrooms that have lots of places for dirt to hide, such as chanterelles and hedgehogs, submerge in a bowl of cool water and gently agitate with you hands to loose any particles. Drain, rinse carefully under running water, and gently pat dry with a towel.