In a 12-inch skillet, heat butter and oil over medium-high heat. Add potatoes and cook, stirring occasionally, until lightly browned, 8–10 minutes. Add onions and cook, stirring occasionally, until soft, about 10 minutes.
Add beef, garlic, thyme; cook, stirring occasionally, until beef browns, about 5 minutes. Season with salt and pepper to taste.
Add heavy cream, stir to combine, and press hash down into the skillet with a metal spatula. Turn hash in parts every 2 minutes, loosening any browned bits, until the cream has reduced and hash forms a crust, about 10 minutes. Garnish with parsley or chives and serve hot with fried or poached eggs, if you like. Yield: 4 Servings.