Steps:

• Heat up the oil in a skillet over medium heat. Add the onions and saute until translucent. Set aside to cool down.
• In a large mixing bowl, combine the ground sirloin and onions. Season with salt and pepper, then mix until evenly incorporated.
• Cover, then store in a chilled area for at least 30 minutes. This will help keep the proteins cold, ensuring that the patties will hold shape while being formed.
• Preheat the grill and grease with cooking spray. Preheat the oven to 350 degrees F.
• Form roughly 7-ounce patties using both your hands and a mold of your choice. Once formed, grill the burgers for roughly 5 minutes on each side, then roast in the oven for another 5 minutes or up to your desired doneness.
• Slice the hamburger buns in half and warm by either toasting or grilling.
• Assemble the sandwich. Start by spreading roughly 2 tablespoons of mayonnaise on both top and bottom buns. Place the lettuce at the bottom bun, then layer with the cooked patty, tomatoes, pickles, and finally cover with the top bun.
• If using, skewer the burgers to hold these in place. Serve immediately while still hot, either with onion rings or potato fries.

Steps:

• Combine all sauce ingredients in small bowl; set aside. 
• Gently combine ground beef. (Do not overmix.) Divide into 4 equal portions. Gently shape into 4 1/2-inch patties, each about 3/4 inch thick. Place onto parchment-lined platter. Create indent in top of each patty by pressing 3 fingers into center. (The indent should be about 1/4 inch deep and 3 inches wide.) Season both sides of each patty with salt and pepper. Refrigerate until grill is ready. 
• Heat one side of gas grill on medium or charcoal grill until coals are ash white. Place coals to one side in charcoal grill.
• Place burgers onto grill away from heat, indent-side up. Close grill; cook 15-16 minutes or until internal temperature of center of burger reaches 145°F.
• Meanwhile, cook bacon in 10-inch skillet over medium heat until sizzling and almost cooked. Add mushrooms and soy sauce. Cook, stirring often, until mushrooms are golden brown. Set aside; keep warm.
• Transfer burgers to heated side of grill. Cook 1-2 minutes or until bottom side is charred. Flip burgers; top each burger with 2 slices cheese. Continue cooking until cheese is melted and thermometer inserted into center reaches 160°F or until desired doneness. 
• Spread bottom of each bun with about 1/2 tablespoon of sauce, place burger on bun, top with about 1 tablespoon sauce, mushrooms, bacon and 2 onion rings.