Put the olive oil and balsamic vinegar in a pitcher. Add the basil, parsley, thyme, salt and pepper and whisk together. Pour the mixture into a resealable plastic bag. Add the mushrooms, seal the bag and toss around to coat. Grill immediately, or leave the mushrooms in the marinade for up to 2 hours.
When ready to cook, heat a grill or grill pan to medium high. Cook the mushrooms until lightly charred and tender, 4 to 5 minutes per side. Toast the buns cut-side down until golden, a couple of minutes.
Serve the mushrooms in the buns with sliced tomato, red onion, butter lettuce, mustard, mayonnaise and ketchup.
Copyright 2015 Television Food Network, G.P. All rights reserved.