Preheat oven to 200 °C. Finely chop or purée one clove of garlic and mix it with 2 tbsp olive oil, salt and pepper. Wash mushrooms and cut off the stem and pierce the cap with a fork several times. Coat the mushrooms with the marinade and grill in oven for about 5 min on each side. Meanwhile, julienne the onion, slice garlic and mushroom stems and heat oil on a pan, caramelize onion with garlic, add mushroom stems, wine, basil and olive powder and stir constantly until wine evaporates. Arrange the burgers and pour the onion garnish on top.