I LOVE these burgers! I will order this in restaurants and wanted to make them at home. The caramelized onions really add a great flavor layer to these. Also, a nice alioli perfects this! I am going to try this with a tomato jam next. That way, the tomato wont keep falling out when you eat it! :)
Combine the vinegar,olive oil,basil, oregano,and garlic together. Place mushroom caps smooth side up in a shallow baking dish. Pour marinade over the mushrooms and let stand at room temp. for 15 minutes, turning the mushrooms over 2 or 3 times. The idea is not to drown the mushrooms, only coating them with a thin marinade to add flavor. Do not put them in a zip lock, as they will get soggy. Just brushing the marinade will do.
To caramelize the onions, take your very thin slices of onion and add them to a frying pan with olive oil. Heating the oil first till it is hot. Then add your pinch of brown sugar, and over a medium low heat, caramelize the onions until they are transparent. I also like to add just a touch of the marinade the mushrooms are marinating in.
You can grill these mushroom caps (oil the grill first to prevent sticking) or broil them at the lowest position away from heat source. Cook them until the edges just start to brown up (about 3-5 minutes) and then turn them and cook an additional 3-5 minutes. If you broil them, pat them with paper toweling after you remove them form the oven, to prevent them from dripping.
I like to place a nice slice of cheese on the bun. mozzarella or provolone add much flavor to this. Then place the hot mushroom on top of cheese slice on bun. Then a slice of tomato, a bit of lettuce if you like, your caramelized onions and the alioli. Enjoy!