Mix meat, stuffing mix, 1-1/4 cups water and onions just until blended. Shape into 6 (1/2-inch-thick) oval patties.
Place on rimmed baking sheet.
Bake 25 min. or until done (160F). Meanwhile, cook mushrooms in large nonstick skillet on medium-high heat 5 min. or until tender, stirring occasionally. Stir in barbecue sauce and remaining water; simmer on low heat 1 to 2 min. or until heated through.