Heat butter and oil in a large skillet over medium-high heat. Add beef stew meat and cook until browned, 5 to 8 minutes. Brown beef in batches if necessary, making sure to not overcrowd the pan, otherwise the beef won't brown. Remove browned beef from the skillet and place in the slow cooker.
Add onion to the skillet. Cook and stir until softened and beginning to take some color, 3 to 5 minutes. Transfer onion to the slow cooker.
Place skillet back on medium-high heat and pour in half of the wine. Bring the wine to a simmer, scraping up any browned bits from the bottom of the pan. Remove from heat and carefully pour into the slow cooker along with the remaining wine. Add carrots, thyme, bay leaves, salt, and pepper.
Place the lid on the slow cooker and cook on Low for 6 to 8 hours.