Blue Cheese Portobello Burgers

** Prep time includes marinating.

For the burgers, mix the vinegar, oil, garlic, rosemary, salt, and pepper in a shallow dish wide enough to fit the portobellos in an even layer (or you could do this in a resealable plastic bag). Let the mushrooms marinate for 1-2 hours making sure to turn the mushrooms once or twice.

In a large skillet over medium heat, melt the butter and oil and add the sliced onions. Add the salt and sugar. Cook over medium-low heat, stirring occasionally. Don’t let the onions brown too quickly, the whole process should take about 30 minutes or until the onions are evenly dark golden brown.

Preheat the grill. Remove the mushrooms from the marinade and place gill sides-down on the grill. Cook for 5 minutes, then turn. Mound the blue cheese on the gills of each mushroom and continue to cook until tender and the cheese is soft and melty, about 6 to 8 minutes. Open the rolls and lightly toast on the grill.

Place the portobellos on a roll and top with caramelized onion, some lettuce, and the bun top. Devour with gusto!