Portobellos are my favorite choice when going to BBQs or inviting some friends over. They are extremely delicious, juicy and adaptable to any kind of add-on that you can think of. Just get creative by adding different seasonings, vegetables or even some melted cheese.
Prepare the Portobello mushrooms by removing the stalks and cleaning them with a damp towel.
Mix all the marinade ingredients together. Using a cooking brush, brush the mushrooms with the sauce on both sides. Let them marinate for 30 minutes.
Place onions in a large skillet with the 2 Tablespoons of olive oil, cook them at medium heat until golden brown. The onions will need between 10-15 minutes to be caramelized. Stir regularly; otherwise, it will burn.
To make the pesto mayonnaise, blend all the ingredients until smooth. You can use either a stick blender, regular blender, or food processor. Adjust salt to taste.
Cook the Portobellos cap side down in a heated skillet, grill or BBQ until tender. It will take about 8-10 minutes. Mushrooms should be tender when pressed in the center.
Assemble the burgers by layering pesto mayonnaise onto the bread, then the caramelized onions, a juicy Portobello, and a slice of tomato.
Notes
Remove the stalk from the mushrooms with a knife. To clean the mushrooms, use a damp towel or paper towel to gently wipe them clean. Do NOT wash the mushrooms as they will soak up the water.If desired, you can use a spoon to scrape the gills out from the underside of the mushroom. However, for this use, it is not necessary.Other topping ideas: