HEAT oven to 375°F.
MIX meat, stuffing mix, 1 cup water and onions until well blended. Shape into 6 (1/2-inch-thick) oval patties. Place in 17x11-1/2x3/4-inch pan.
BAKE 25 min. or until done (160°F). Meanwhile, cook mushrooms in large nonstick skillet on medium-high heat 5 min. or until tender, stirring occasionally. Add barbecue sauce and remaining water; simmer on low heat 1 to 2 min. or until heated through.
SERVE meat patties topped with sauce.