In a slow cooker, combine mushrooms and pearl onions.
Stir in 3/4 cup of broth and next 7 ingredients (wine through bay leaf); add chicken legs. Cover and cook on LOW for 7 to 8 hours or on HIGH for 3 1/2 to 4 hours.
Using a slotted spoon, transfer chicken and vegetables to a serving platter and keep warm. Discard bay leaf.
Prepare sauce: Skim fat from cooking liquid. Measure 2 cups of cooking liquid (adding water if necessary to equal the 2 cups); transfer to a medium saucepan. In a small bowl, combine remaining broth and flour to a smooth paste (no lumps!); stir into the liquid in the saucepan and cook over medium heat until thickened and bubbly; cook and stir for 1 minute more. Serve sauce over chicken.