Prepare the lemon mayo: Combine mayonnaise, lemon zest, lemon juice, and lemon pepper in a small bowl. Mix well and set aside.

Prepare the marinade: Combine balsamic vinegar, olive oil, garlic cloves, and chopped basil in a resealable plastic bag.

Add the Portobello mushroom caps to the marinade bag, ensuring they are well coated.

Refrigerate the mushrooms in the marinade for 1 hour.

Prepare the red onion slices: Drizzle them with olive oil.

Grill the onion slices until softened and slightly charred.

Remove the marinated mushrooms from the bag.

Pat the mushrooms dry with a paper towel to remove excess marinade. Discard the garlic cloves and basil pieces stuck to the mushrooms. Season the mushrooms with salt and pepper.

Place the mushrooms gill-side down on the grill.

Cook for 5 minutes.

Flip the mushrooms over.

Arrange crumbled blue cheese on the gill side of each mushroom. Cook for an additional 6-8 minutes, or until the cheese is melted and bubbly.

While the mushrooms are grilling, toast the whole wheat rolls.

Assemble the burgers: Place one cheesy Portobello mushroom on each toasted roll, top with grilled onions, and garnish with fresh basil leaves.

Add lemon mayo to taste.