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PREPARE stuffing as directed on package.
MEANWHILE, melt butter in large nonstick skillet on medium heat. Add mushrooms; cook and stir 3 min. or until tender. Place in large bowl. Add prepared stuffing, tuna, cheese and pepper; mix well. Shape into four patties. Wipe skillet clean with paper towels. Spray with cooking spray.
ADD patties to skillet; cook on medium heat 3 to 4 min. on each side or until heated through. Serve in buns.