Gently wash and dry mushroom caps. Place them ribbed side up on a rimmed baking sheet.
Whisk together olive oil and balsamic vinegar in a small bowl.
Brush half of the oil/vinegar mixture on the ribbed sides of the mushrooms. Season with salt and pepper, to taste.
Broil for 5 minutes.
Flip the mushrooms and brush on the remaining oil/vinegar.
Broil for another 5 minutes.*
Assembly for one burger
Spread softened goat cheese on the bottom bun.
Top with spinach, then portobello mushroom.**
Top that with some caramelized onions and 1 tablespoon of pesto.
Top with burger bun, repeat with the remaining burgers and enjoy!
*Some of the oil/vinegar will roll off the mushrooms and may start to smoke on the pan. Don't worry...it's totally normal!**After the mushrooms were cooked, Trevor cut the mushrooms to fit the size of bun we have on hand. This isn't necessary, but makes for a really pretty presentation!***Gluten free? Make sure to use a GF bun! :)