Mix ingredients for the marinade. Clean mushrooms, brushing them lightly. Lay in a pan gill side up, pour marinade over the caps, and set aside for 30 minutes.
If using an electric grill, lay mushrooms on the grill. If not using a grill, once mushrooms are finished marinating, place them in a skillet, and sliced onions to the centre of the pan. Cook until mushrooms are soft, turn once and put a weighted pan to get grill marks.
Once mushrooms are fully cooked, add sauce to buns, layer the mushrooms, caramelized onions, spinach, and tomato, and serve.