Step 1: Mix together the balsamic vinegar, oil and garlic powder in a bowl big enough to hold mushroom.
Step 2: Place the mushroom gill side up in the bowl and let sit for 1/2 hour.
Step 3: Place the mushroom gill side up on BBQ grill along with the onion slice, over high heat.
Step 4: Grill onion on both sides until it is slightly charred.
Step 5: Grill mushroom about 3 to 5 minutes until you have nice grill marks then turn it over and grill for another 3 to 5 minutes.
Step 6: Place the mushroom on bottom bun; add onion, lettuce and pesto sauce.
Step 7: Add top bun and serve.
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