Grilled Portobello Mushroom Sandwich with Red Pepper Garlic Mayo

4 servings

Ingredients

4 fresh Portobello mushrooms, cleaned
2 tablespoons balsamic vinegar
2 tablespoons olive oil
1/2 teaspoon salt
1/2 teaspoon pepper
4 thick slices sweet onion
4 cups fresh baby spinach leaves, washed and dried
1 large ripe tomato, sliced
4 slices low-fat Swiss cheese
4 whole-wheat hamburger buns

Ingredients for mayonnaise

1 clove fresh garlic
1/2 cup chopped and drained roasted red pepper
2 tablespoons rice wine vinegar
1/4 teaspoon cayenne pepper
1/4 cup low-fat mayonnaise
Salt and pepper, to taste

Directions

Preheat grill (or broiler) to medium heat and spray with oil spray. Skip to step 3 if you do not plan to use the mayonnaise.
Prepare the mayonnaise first. Put all the mayonnaise ingredients in a food processor and purée until smooth. Season to taste with salt and pepper. Cover and refrigerate for at least 30 minutes.
Remove the stems and gills (the dark part of the underside of the mushroom cap) from the Portobello mushrooms to expose the white meat. Then peel the caps using a paring knife to remove the paper-thin, dark exterior. Mix vinegar, oil, salt and pepper. Brush mixture onto the mushrooms, and allow to stand at room temperature for at least 5 minutes.
Grill or broil mushrooms and onion slices until both are soft, well-marked and lightly brown, about 5 to 8 minutes, turning on each side. You can also bake at 450° for about 8 minutes. If mushroom and onions are cooked ahead of time, hold at room temperature until sandwich is ready, or refrigerate. Reheat before assembling.
To prepare the sandwiches, toast the buns until lightly colored and soft and place on plate. Spread 1 tablespoon roasted red pepper mayonnaise (optional) on the bottom halves of the bun. Top with spinach, mushroom, slices of tomato, grilled onion and a slice of cheese.