Preheat grill to medium-high heat.
Combine ground sirloin and steak seasoning in mixing bowl. Mix well with your hands. Divide into 6 sections. Form a small round ball with your hands until tight. Then flatten with both hands to form patty.
In a small skillet, melt butter. Cook onion until translucent, 3-5 minutes. Add garlic, mushrooms, olive oil and Worcetershire sauce. Season with salt and pepper, to taste. Simmer until mushrooms are soft and dark, approximately 15 minutes.
Meanwhile, grill burgers 6-8 minutes per side.
Serve burgers on hamburger buns topping with mushrooms and any other desired toppings like lettuce, tomato, cheese, mayonnaise and ketchup.