Created by lazyme on April 25, 2019
Step 1: Heat gas grill on medium or charcoal grill until coals are ash white.
Step 2: Meanwhile, melt butter in 10-inch skillet until sizzling; add mushrooms. Cook over medium heat until mushrooms are lightly browned (5 to 8 minutes). Remove from heat. Keep warm.
Step 3: Shape ground beef into 4 (3/4-inch thick) patties.
Step 4: Place patties on grill. Grill, turning once, until cooked through (15 to 18 minutes).
Step 5: To serve, spread 1 tablespoon mustard on cut-sides of bun. Place 1 burger on bottom half of bun; top with 1/4 of the mushrooms, 1 slice cheese and top half of bun. Repeat with remaining ingredients.