Place flour in a small shallow dish. Add salt, pepper and garlic powder. Dredge meat in flour, shaking off any excess. Add 2 Tablespoon olive oil to a large frying pan. Brown meat. (May need to do in batches.) Remove meat and place on a paper towel lined plate. Add the remaining 1 Tablespoon of olive oil  + butter to the pan. Add in onions. Saute for 3 minutes. Add mushrooms. Saute for two minutes more. Add meat, onion, and mushrooms to a slow cooker. Add in potatoes, diced tomatoes, carrots, and celery, and tomato paste.  Pour in beef broth. Add thyme and bay leaves.  Cook on low for 6-8 hours or high for 4-6 hours. 30 minutes before serving, make a thickening slurry by whisking together cornstarch and water. Pour into the stew and stir well.