1 1/2 cups sliced onions (about 3 medium)
8 oz. button or crimini mushrooms, sliced (3 cups)
1 tbsp. olive oil
2 tbsp snipped fresh parsley
2 1/2 lb. 85% lean ground beef
1/3 cup Worcestershire sauce
4 cloves garlic, minced
1 1/2 tsp. salt
1 tsp. ground black pepper
16 slices apple wood smoked bacon (optional)
8 slices Swiss, provolone, and/or Colby and Monterrey Jack cheese (optional)
8 Kaiser rolls or hamburger buns, split and toasted
Tomato slices (optional)
1. In very large skillet cook onions and mushrooms in hot oil over medium heat 15 to 20 minutes or until all liquid has evaporated, and onions and mushrooms are tender and golden brown, stirring occasionally. Remove from heat. Stir in parsley; set aside to cool slightly.
2. In very large mixing bowl mix beef, Worcestershire sauce, garlic, salt, and pepper until just combined (do not over mix). Shape mixture into 16 (3 1/2-inch-diameter) patties. Spoon filling onto half of patties to 1/2-inch of edge. Top with remaining patties; pinch to seal. If using, lay 2 bacon slices over each stuffed patty in crisscross pattern, tucking ends under.
3. For charcoal grill, arrange preheated coals around drip pan. Test for medium heat above pan. Place patties over drip pan on grill rack. Cover and grill 25 to 30 minutes or until done (160°F). (For gas grill, preheat grill. Reduce heat to medium. Adjust for indirect grilling. Grill as above.)
Top with cheese. Cover; cook 1 to 2 minutes more or until cheese melts.
4. Transfer burgers to serving platter. Serve on buns with tomato slices.