Vegetarian Roasted Portobello Sandwiches

4 4-inch to 5-inch portobello mushrooms, stems removedOlive oil cooking spray4 thin slices red onion8 thin slices crusty whole-grain bread4 thin slices reduced-fat Swiss cheese (4 ounces)2 jarred roasted red peppers, drained, sliced (about 2 ounces)4 slices tomato4 fresh basil leavesVinaigrette1 tablespoon balsamic vinegar1 tablespoon Dijon mustard3 tablespoons chopped fresh basil1 scallion, white part and 1 inch of the green, finely chopped 1/8 teaspoon garlic powderFreshly ground pepperBalsamic mayonnaise1 tablespoon light mayonnaise2 teaspoons balsamic vinegar1 teaspoon Dijon mustardHeat oven to 450 degrees.Coat the mushrooms with cooking spray. Mix the vinaigrette ingredients and paint the mushrooms on both sides. Marinate for 15 minutes.Coat the marinated mushrooms and the onion slices with cooking spray. Roast them in the oven until cooked through, about 5 minutes.Coat the bread with the cooking spray and toast in a toaster oven or under the broiler.Place a slice of cheese on four of the pieces of toast. Top with a mushroom, slices of onion, red pepper and tomato and a basil leaf.Combine the mayonnaise, vinegar and mustard. Paint each top slice of bread with one-fourth of the balsamic mayonnaise. Assemble the sandwiches and serve immediately.Serves four.Nutrition values per serving: 250 calories, 5 g fat (1 g saturated), 37 g carbohydrates, 6 g fiber, 18 g protein, 8 mg cholesterol, 737 mg sodium."Cleveland Clinic Healthy Heart: Lifestyle Guide and Cookbook" by Bonnie Sanders Polin and Frances Towner Giedt (Broadway Books, $29.95)