HEAT oven to 375ºF.
MIX meat, stuffing mix, 1-1/4 cups water and onions until well blended. Shape into 6 (1/2-inch-thick) oval patties. Place on 15x10x1-inch pan.
BAKE 25 min. or until patties are done (160°F).
MEANWHILE, spray large nonstick skillet with cooking spray. Add mushrooms; cook on medium-high heat 5 min. or until lightly browned, stirring occasionally. Add barbecue sauce and remaining water; simmer on low heat 1 to 2 min. or until sauce is heated through. Serve over meat patties.