Ingredients

1/4 cup vegetable broth

2 tablespoons olive oil

2 teaspoons balsamic vinegar

1 teaspoon chopped fresh thyme leaves

1/4 teaspoon salt

1/4 teaspoon coarsely ground black pepper

4 medium (about 4 ounces each) portobello mushrooms, stems removed

1 lemon

1/3 cup mayonnaise

1 small green onion, trimmed and minced

4 large (about 4-inch) buns

1 bunch arugula, trimmed

Preparation

  1. In glass baking dish just large enough to hold mushrooms in a single layer, mix broth, oil, vinegar, thyme, 1/8 teaspoon salt, and 1/8 teaspoon pepper. Add mushrooms, turning to coat. Let stand, turning occasionally, 30 minutes.
  2. Prepare grill.
  3. Meanwhile, from lemon, grate 1/2 teaspoon peel and squeeze 1/2 teaspoon juice. In small bowl, stir lemon peel and juice, mayonnaise, green onion, and remaining 1/8 teaspoon salt and 1/8 teaspoon pepper.
  4. Place mushrooms on hot grill rack over medium heat and grill, turning occasionally and brushing with remaining marinade, until mushrooms are browned and cooked through, 8 to 10 minutes per side.
  5. Cut each bun horizontally in half. Spread cut sides of buns with mayonnaise mixture; top with arugula leaves. Place warm mushrooms on bottom halves of buns; replace top half of buns to serve