Place onions and mushrooms on a lightly greased grill rack and toss with 1/4 cup balsamic vinegar, olive oil and pepper, mixing well. In a separate bowl, combine remaining balsamic vinegar and Worcestershire sauce; set aside for later use.
Divide meat mixture into 8 equal portions, shaping each into a 1/4-inch to 1/2-thick patty. Brush both sides of patties with reserved Worcestershire mixture and season with lemon pepper.
Grill onions and mushrooms 10 to 12 minutes, until the onions are tender. Place patties on grill rack; grill for 5 minutes per side or until desired degree of doneness. Serve sliders on buns. Top with balsamic onions, mushrooms and desired toppings. Serve immediately.
Notes:
-You can save time if you purchase already-prepared sliders. Some groceries have them in the meat section.
-Purple onions also work great!
-You can roast mushrooms and onions on a lightly greased baking sheet instead of the grill on 400 for 10 to 12 minutes.