Portobello Steak Burger With Caramelized Onions

• 1 cup balsamic vinegar
• 2 tablespoons olive oil
• 1 teaspoon dried basil
• 1/4 teaspoon kosher salt
• 1/4 teaspoon ground white pepper
• 2 garlic cloves, minced
• 6 portobello mushroom caps (about 1 1/2 pounds)
• Cooking spray
• 1 tablespoon olive oil
• 1 1/2 cups sliced onion
• 1/4 cup low-fat mayonnaise
• 1 teaspoon bottled minced roasted garlic
• 6 (2-ounce) whole-wheat Kaiser rolls or hamburger buns
• 6 curly leaf lettuce leaves
• 6 large (1/4-inch-thick) slices tomato

1. Combine first 6 ingredients in a large zip-top plastic bag. Add mushroom caps; seal and marinate in refrigerator 24 hours.
2. Prepare grill or broiler.
3. Remove mushroom caps from bag, discarding marinade. Place mushroom caps on grill rack or broiler pan coated with cooking spray; cook 5 minutes or until tender.
4. Heat the oil in a medium nonstick skillet over medium-high heat. Add onion; cover and cook 10 minutes or until golden brown, stirring frequently.
5. Combine mayonnaise and roasted garlic in a small bowl. Spread 1/2 tablespoon mayonnaise mixture evenly over the top halves of buns; top each bottom half of bun with 1 lettuce leaf, 1 tomato slice, 1 mushroom cap, 1 tablespoon caramelized onion, and top half of bun.

balsamic vinegar, olive oil, basil, kosher salt, ground white pepper, garlic, portobello mushroom caps, cooking spray, olive oil, onion, lowfat mayonnaise, garlic, kaiser rolls, curly leaf, tomato

Taken from www.myrecipes.com/recipe/portobello-steak-burger-with-caramelized-onions (may not work)