Form the burger into two patties slightly larger than the diameter of the mushroom caps. Season with smoked sea salt and fresh ground pepper. Set to the side.
Combine canola oil, vinegar, roasted garlic, sea salt & fresh ground pepper in a small bowl. Brush top and bottom of mushroom cap with vinaigrette.
Heat a CLEAN, OILED grill to 450+ degrees using all the burners. You can cook the mushroom caps and the burgers at the same time and keeping the grill cover closed while they are cooking. Cook the mushroom caps for 5 minutes per side or until grill marks appear. Cook the burgers 5 minutes on the first side and 4 minutes on the second side. Timing may vary depending on the size of your burger. The internal temperature should reach 145 degrees for medium rare.
Building Your Burger: Portabello Mushroom Cap (bottom side up), Burger, Cheese, Leaf Lettuce, Red Onion, Condiment (optional), Portabello Mushroom Cap (top side up)