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Heat a large skillet over medium or medium low heat.
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When hot, add butter.
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Once butter has melted, add sliced onion and cook for about 1520 minutes, or until golden brown and caramelized.
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In the last minute or two, deglaze pan with a couple splashes of dry white wine.
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Cook until wine has absorbed.
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Set aside.
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Preheat broiler (ours is at 500 degrees F).
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Gently wash and dry mushroom caps.
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Place them ribbed side up on a rimmed baking sheet.
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Whisk together olive oil and balsamic vinegar in a small bowl.
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Brush half of the oil-vinegar mixture on the ribbed sides of the mushrooms.
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Broil for 5 minutes.
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Flip mushrooms and brush on remaining oil-vinegar.
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Broil for another 5 minutes.
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Remove from the oven and sprinkle with a little salt.
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To assemble burgers, spread softened goat cheese on the bottom bun.
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Top with spinach, then portobello mushroom.
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Top that with some caramelized onions and 1 tablespoon pesto.
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Top with burger bun.
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Notes: 1.
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Some of the oil and vinegar will roll off the mushrooms and may start to smoke on the pan.
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Dont worry, its totally normal!
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2.
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After mushrooms were cooked, Trevor cut the mushrooms to fit the size of bun we have on hand.
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This isnt necessary, but makes for a really pretty presentation!
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3.
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Gluten-free?
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Make sure to use a gluten-free bun!
