Grilled Portabella Burger with Fontina and Basil

Preheat grill or broiler.

In a small bowl whisk together lemon juice, olive oil, garlic, salt and 1/4 teaspoon black pepper. Brush both sides of each mushroom with lemon juice mixture, using all of the dressing. Place mushrooms on a grill or on a broiler pan rack, rounded-side up; cook for 3 minutes. Turn mushrooms stem-side up; sprinkle with cheese, oregano and remaining 1/2 teaspoon black pepper. Continue cooking until mushrooms are tender and cheese is melted, about 3 minutes.

To serve, place each mushroom into a roll and sprinkle with basil. Top with sliced tomato and red onion, if desired. Serve immediately.