Portobello Steak Burger with Caramelized Onions

Ingredients

• 6 curly leaf lettuce leaves
• 6 large (1/4-inch-thick) slices tomato
• 2 tablespoons olive oil
• ¼ cup low-fat mayonnaise
• 1 teaspoon bottled minced roasted garlic
• 1 cup balsamic vinegar
• ¼ teaspoon ground white pepper
• + 8 more ingredients
• • Cooking spray
  • 6 (2-ounce) whole-wheat Kaiser rolls or hamburger buns
  • 1 teaspoon dried basil
  • ¼ teaspoon kosher salt
  • 2 garlic cloves, minced
  • 6 portobello mushroom caps (about 1 1/2 pounds)
  • 1 tablespoon olive oil
  • 1 ½ cups sliced onion

Combine first 6 ingredients in a large zip-top plastic bag. Add mushroom caps; seal and marinate in refrigerator 24 hours. Prepare grill or broiler. Remove mushroom caps from bag, discarding marinade. Place mushroom caps on grill rack or broiler pan coated with cooking spray; cook 5 minutes or un...

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