Heat a large skillet over medium-high heat. Add beef, mushrooms, and onion. Cook and stir until beef is browned and crumbly, 5 to 7 minutes. Drain and discard grease. Transfer mixture to a bowl. Add garlic and oil to the skillet; saute until garlic is golden brown, about 1 minute.

Turn skillet heat to high; add red wine and beef stock. Bring to a boil while scraping the browned bits off of the bottom of the pan with a wooden spoon. Reduce heat and simmer until flavors meld, about 5 minutes.

Whisk sour cream and cornstarch together in a bowl. Mix into the wine/broth mixture. Stir in the browned beef mixture. Bring to a simmer, stirring frequently. Add salt and pepper.