Heat butter and oil in a large skillet over medium-high heat. Add beef and cook, stirring occasionally, until browned on all sides, 5 to 8 minutes. Brown beef in batches if necessary, making sure not to overcrowd the pan; otherwise, beef will not brown properly. Remove browned beef to the slow cooker.
Add onions to the same skillet. Cook and stir until softened and beginning to take some color, 3 to 5 minutes. Transfer onions to the slow cooker.
Place the skillet back over medium-high heat and pour in 1/2 of the wine. Bring to a simmer, scraping up any browned bits from the bottom of the pan with a wooden spoon. Remove from heat and carefully pour into the slow cooker along with remaining wine. Add carrots, thyme, bay leaves, salt, and pepper.
Cook on Low for 6 to 8 hours.
