Rinse beef and pat dry. Cut across grain into 0.125" thick slices.
Pour 1 Tbs olive oil into frying pan medium-high; when hot add beef and stir until browned on edges but still pink in center (2-3 minutes). Pour into another bowl.
Add remaining 2 Tbs olive oil into pan, when hot add onion and garlic, stirring until the onion is limp (3-5 minutes). Add mushrooms and stir often until beginning to brown (3-5 minutes).
Add wine, cognac, and oyster sauce to mushroom mixture. Bring to a boil, simmer 5 minutes. Add cream and return to simmer. Mix cornstarch with 1 tsp cold water, mix until smooth and then add to mushroom mixture. Stir 2 minutes until mixture thickens.
Add sour cream and beef with any accumulated juices and stir until heated through, about 1 minute.