Place beef in slow cooker. Mix together flour, salt, and pepper in a small bowl. Pour over meat and stir to coat. Stir in garlic, bay leaf, paprika, Worcestershire sauce, onion, beef broth, potatoes, carrots, celery, and butternut squash. Cover and cook on Low for 10 to 12 hours or on High for 4 to 6 hours. Stir in escarole. Cover and cook until escarole is tender, 15 to 20 minutes.