In a large skillet over medium high heat, melt the butter in the oil. When foaming subsides, add the beef, taking care not to overcrowd the pan. Brown beef on all sides, in batches if necessary. Add garlic when browning the last batch. Transfer beef to slow cooker as it is browned.
Add remaining ingredients to slow cooker. Cover and cook on low 6-8 hours or high 3-4 hours. If sauce is thin, just before serving, add a little cornstarch dissolved in cold water.