To prepare garlic aioli- in a small bowl mix together mayonnaise, fresh garlic, lemon juice, and lemon zest. Season with salt and pepper. Cover and refrigerate until needed. Heat oil in a deep fryer to 375 degrees. Add onion rings and fry in oil until golden brown. Drain on paper towels. Set aside.
Heat olive oil and butter in a medium skillet over medium heat. Add mushrooms and season with salt and pepper. Sautee for about 4 minutes. Add Worcestershire sauce. Sautee the mushrooms for about 20 minutes or until the mushrooms are tender.
Lightly grease the grill grates of an outdoor grill, and heat the grill to medium-high heat. Form ground beef into 4 equal patties. Make a depression in the center of each patty with your thumb. Season the patties on both sides with Montreal steak seasoning. Place hamburger patties onto the grill and cook, turning once until the internal temperature has reached 160 degrees. During the last few minutes of cooking, top each hamburger patty with a slice of Swiss cheese. Once the cheese has melted remove the cooked hamburger patties from the grill.
While hamburgers are grilling, split the hamburger buns and place them cut side down onto the grill. Cook until lightly toasted. To assemble burgers- place each hamburger patty onto the top of the bottom half of each bun. Top with mushrooms, bacon, and onion rings. Spread the garlic aioli onto the bottom of the top half of the bun. Place the top half of the bun onto the burger.