1. Trim stem off of portobello mushrooms and wipe dirt off of the mushrooms with a wet paper towel.
2. In bowl, mix together olive oil, lemon juice, garlic, salt, and pepper. Place mushrooms on plate gill-side-down and brush lightly with olive oil mixture. Flip the mushroom and brush the gill side with remaining mixture. Let sit ten minutes.
3. Preheat grill to medium heat. Add mushrooms gill side up and grill until nice grill marks are underneath, about four minutes. Turn the mushrooms 90° to create grill marks and cook two minutes longer. Flip and repeat. Flip mushrooms again and place 1 ounce of Brie on the mushroom and allow to melt. Take off grill and place on plate.
4. In small bowl combine mayonnaise and pesto. Spread 1 tablespoon of mixture on each bun. Add mushroom to bun. Top with your favorite burger toppings.

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