For the Garlic Mayonnaise:
5 garlic cloves
1 cup olive oil
½ cup mayonnaise
Kosher salt, to taste
For the Burger:
1 pound ground sirloin
¼ pound ground pork
Kosher salt and freshly ground pepper, to taste
1 small red onion, sliced ¼ inch thick
½ cup apple cider vinegar
¼ cup olive oil
3 ounces Brie cheese, sliced into 4 thin pieces
4 pretzel rolls, split in half
2 cups watercress
1. Make the garlic mayonnaise: In a small saucepan over medium-low heat, bring the garlic and olive oil to a simmer; cook until the garlic is soft but not brown, 15 to 20 minutes. Using a slotted spoon, remove the garlic and reserve the garlic-infused olive oil for another use.
2. In a small bowl, using a metal spoon, smash the garlic; stir it into the mayonnaise, season with salt and set aside. Cover and chill for at least 30 minutes. Make ahead: Garlic mayonnaise can be made, covered and chilled for up to 5 days.
3. Make the burger: Using your hands, mix the sirloin and pork together, and season generously with salt and pepper. Form into 4 patties, each about ½ inch thick. Refrigerate until ready to grill.
4. In a small bowl, toss the onions with the vinegar and olive oil. Season with salt and pepper and let the onions marinate for at least 30 minutes or preferably overnight.
5. Prepare a charcoal or gas grill for medium-hot heat or place a grill pan over medium-high heat.
6. Remove the onions from the marinade and place on the grill. Cook until lightly charred, turning occasionally, about 3 to 4 minutes on each side. Set the onions aside and let cool slightly. Place the patties on the grill and cook until lightly charred on the bottom, 5 to 6 minutes. Flip and top each patty with 1 slice of Brie. Grill to desired doneness, about 4 to 5 more minutes for medium. Grill the buns for 1 minute until lightly charred.
7. Spread the garlic mayonnaise on cut sides of the buns. Top with the cheeseburger and then garnish with the onions and some watercress. Serve immediately.