4 portobello mushrooms (choose a size that will fit on burger buns)
1/3 cup GF soy sauce such as the VH brand
1/3 cup rice vinegar
2 teaspoons sesame oil
2 cloves garlic, minced
1 teaspoons fresh grated ginger
1 tablespoon agave nectar or liquid honey
1/4 teaspoon freshly ground pepper
4 hamburger buns
1/4 cup GF sesame mayonnaise
4 lettuce leaves
8 slices cucumber
Wipe the portobello mushrooms clean and carefully remove and discard the stems.
Place the mushrooms in shallow glass dish.
Combine next seven ingredients in a bowl and brush over the top of mushrooms to coat.
Turn the mushroom over, gills facing up, and pour the remaining marinade over the mushroom.
Turn the mushroom over a couple of times to coat and refrigerate several hours or overnight.
Grill or broil, 3 to 4 minutes per side, or until tender, basting frequently with marinade.
Serve whole on toasted buns topping each with 1 tablespoon sesame mayonnaise, a leaf of lettuce and couple of slices of cucumber.