1. Combine beef in a large resealable plastic bag with 2 tbsp of the flour, 1/4 tsp of the salt and the pepper. Shake to coat. Melt butter and oil in a large stainless steel skillet over medium-high heat. Add beef and brown for 5 minutes, stirring to sear all sides (cook in 2 batches, if needed). Spoon into slow cooker. Reduce heat under pan to medium and add 1 cup of the broth. Cook for 2 minutes, stirring up any brown bits on bottom of pan. Pour into slow cooker along with carrots, mushrooms, garlic, red wine and remaining beef broth. Scatter onions over top of vegetables.
2. Cover and slow cook on HIGH for 5 hours or LOW for 61/2 hours. With a slotted spoon, remove beef and vegetables to a large serving bowl.
3. Whisk together remaining 3 tbsp flour with 3 tbsp water in a small bowl. Strain hot liquid from slow cooker into a medium saucepan. Whisk in flour mixture and bring to a boil over medium-high heat. Boil 3 minutes, until thickened, then stir into stew with remaining 1/4 tsp salt. Serve stew over mashed potatoes or noodles, if desired.