Heat your grill or barbecue to medium high heat. 

For each mushroom, stir together 1 tablespoon of pesto with 1 tablespoon of mayo (may need slightly less depending on the size of the mushroom).

When grill is hot, toast a whole grain burger bun until golden. Set aside.

Brush pesto mixture over the entire mushroom, top and bottom. Place on grill top side down, so the gills are visible.

Grill mushroom, with lid closed, for approximately 4 - 5 minutes, or until you see juices bubbling in the bowl of the mushroom cap. Do not flip.

If desired, top with a sprinkle or slice of non-dairy parmesan or another sharp vegan cheese. Turn off heat, close the lid, and allow cheese to melt (a minute or so).

Add a smear of mayo to the top half of your burger bun, as well as a few lettuce leaves and slices of tomato. Don't use ketchup or mustard on these, as it will overwhelm the other flavours.

Place your mushroom cap on the bottom half of the bun, keeping top side down to preserve the juices. Cover with the top half of the bun, and grab some napkins!