Portobello Truffle Burger

1.In a bowl, whisk together balsamic vinegar, olive oil, salt, and black pepper. Add portobello mushroom caps, ensuring they are well coated. Marinate for at least 30 minutes, turning occasionally.

2.While the mushrooms marinate, heat a skillet over medium heat. Add butter and sliced onions, cooking slowly until they are caramelized, about 15-20 minutes, stirring occasionally. Remove from heat and set aside.

3.Preheat a grill or grill pan over medium-high heat. Remove mushrooms from the marinade and grill for about 5-7 minutes on each side, or until tender and grill marks appear. In the last minute of grilling, place a slice of Gruyère cheese on each mushroom cap to melt.

4.While the mushrooms are grilling, toast the brioche buns on the grill until golden brown, about 1-2 minutes.

5.In a small bowl, mix together mayonnaise, truffle oil, and lemon juice to create the truffle aioli.

6.To assemble the burgers, spread a generous amount of truffle aioli on the bottom half of each brioche bun. Place the grilled portobello mushroom with melted Gruyère on top, followed by a portion of caramelized onions and a handful of fresh arugula. Top with the other half of the bun.