In a large bowl, toss cubed beef with flour to coat.
Heat 1 tablespoon of butter in a heavy skillet over medium-high heat; add coated beef cubes and brown well on all sides (don’t worry about cooking them through—you just want to brown the outsides); transfer to a slow cooker.
In the same skillet, cook onion and garlic until onion is translucent; transfer mixture to the slow cooker; add water, broth, parsley, salt, pepper and bay leaf. Cover and cook on LOW for 2 hours.
Add potatoes, carrots, celery and bell pepper; cover and cook for another 2 hours, adding more water if needed at this time. Remove bay leaf and add mushrooms; cover and continue to cook for 4 hours or until beef and vegetables are fork-tender.
About 30 minutes before serving time, prepare dumplings: In a small bowl, combine biscuit mix, remaining butter and the milk just until blended.
Drop the dough by the tablespoonful into the stew. Raise slow cooker heat setting to HIGH and cook for another 10 minutes or until dumplings are cooked through, but not dry.