If you have a broiler, this dinner meal can surprise you the first time you make it; it's seriously quick (except for the marination part).
In a bowl mix the vinegar, olive oil, garlic, lemon juice, salt and pepper.
Pour the mixture in a plastic bag and drop the mushrooms(stem removed) in it. Tightly lock the bag and shake the mixture.
Let the mushrooms stand for at least 15 minutes (If you have time, an hour of marination would be great)
In a large baking tray, arrange the onions and the buns.
Add cheese to the buns.
Keep the tray in the broiler and let it cook for a minute.
Remove the buns and let the onions cook for another minute.
If the onions are grilled enough, take them out of the broiler and keep aside.
Take the marinated mushrooms out of the bag and place them on the baking tray.
Let the mushrooms grill for at least 2 minutes on each side (this can vary depending on your broiler).
Once the mushrooms are grilled, take them out and keep one mushroom on one bun each.
Arrange the grilled onions on top, add some Tobasco/Mustard sauce if you like.
Serve the buns with the cucumber and tomatoes on the side; I like to place them in the bun itself.