Place the stew meat, salt, pepper, and onion in the slow cooker. Stir to distribute the seasonings and onion.
In a small bowl, combine the garlic salt, Worcestershire sauce, beef broth and ketchup. Pour over the meat.
Cover and cook for 7-9 hours on low or 4-5 hours on high.
About 30 minutes before serving, combine the flour and apple juice in a small bowl, whisking vigorously to combine well.
Pour the flour mixture into the slow cooker, whisking quickly to mix in the roux with the stew meat and juices and to avoid lumps.
Add the mushrooms and stir.
Cook on high for 30 minutes.
Stir in 1/2 cup sour cream right before serving.
Serve over baked potatoes.