My husband and I had always wanted to try mushroom burgers. We kept buying the mushrooms, but never quite made the burgers. Finally, we made them and just loved them! They are just as satisfying as a hamburger, but not quite as filling.
Combine the ingredients for the burger marinade in a large resealable plastic bag. Add the mushrooms and marinate about 2 hours at room temperature. Discard the marinade.
In a small bowl, combine all the ingredients for the aioli.
Prepare the grill for medium heat. Cook the mushrooms about 4 minutes per side. Meanwhile, toast the cut side of the buns if desired.
To serve: Spread about 2 tablespoons of the aioli on the top half of each bun. On the bottom half of the bun, arrange a grilled portabello, lettuce, and a thick tomato slice. Cover with the top and enjoy.