Portobello Burgers with Pear-Walnut Mayonnaise

Directions

1. In a large resealable plastic bag, combine the oil, vinegar, garlic and thyme. Add mushrooms; seal bag and turn to coat. Refrigerate for up to 2 hours.
2. In a small skillet over medium heat, cook pear in oil and lemon juice until tender. Transfer to a small food processor; cover and process until blended. Stir in mayonnaise and walnuts. Refrigerate until serving.
3. Drain mushrooms and discard marinade. Grill mushrooms, covered, over medium heat or broil 4 in. from the heat for 3-4 minutes on each side or until tender. Top with onion and cheese. Grill 2-3 minutes longer or until cheese is melted. Serve on buns with mayonnaise mixture and arugula.