Nutritional Information Per Serving Calories: 274 Fat: 9.3g Protein: 8.9g Carbohydrates: 40.4g Fiber: 3.7g Cholesterol: 3mg Sodium: 420mg Servings 6 servings. Time To Cook 24 hours marinade time. Remaining cooking time: 20 minutes.  Portobello "Steak" Burger  With Caramelized Onions  Ingredients 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 medium to large size portobello mushroom caps, about 1 1/2 pounds cooking spray of your choice 1 tablespoon olive oil 1 1/2 cups sliced onion 1/4 cup lower-fat mayonnaise 1 teaspoon bottled minced roasted garlic 6 whole-wheat Kaiser rolls or hamburger buns, about 2 ounces each 6 curly leaf lettuce leaves 6 large slices fresh tomato, about 1/4 inch thick  Directions     Combine first 6 ingredients in a large zip-top plastic bag. Add mushroom caps and seal and marinate in refrigerator for 24 hours. Prepare grill or broiler. Remove mushroom caps from bag and discard marinade. Place mushroom caps on grill rack or broiler pan coated with cooking spray and cook 5 minutes or until tender. Heat the oil in a medium nonstick skillet over medium-high heat. Add the onion and cover and cook 10 minutes or until golden brown, stirring frequently. Combine mayonnaise and roasted garlic in a small bowl. Spread 1/2 tablespoon mayonnaise mixture evenly over the top halves of buns - rolls. Top each bottom half of bun with 1 lettuce leaf, 1 tomato slice, 1 mushroom cap, 1 tablespoon caramelized onion. Top with other half of bun - roll.   © 2003 Roberts Food Center. Designed and Hosted by The Computer Company, Inc.