Notes: Buy portabella mushroom caps. Or buy whole mushrooms, and trim off stems and save them for other uses.
2 large egg whites
1 tablespoon olive oil
1/3 cup Italian-style fine dried bread crumbs
2 tablespoons chili powder
4 portabella mushroom caps (5 in. wide; 1 to 1 1/4 lb. total)
4 round sandwich buns (4 in. wide; 3/4 lb. total)
1 cup nonfat sour cream
2 teaspoons minced canned chipotle chilies or hot sauce
1/4 cup thinly sliced red onion
1 cup spinach leaves, rinsed and crisped
How to Make It
In a bowl, beat egg whites and oil to blend. In another bowl, mix crumbs and chili powder.
Rinse mushroom caps; drain. Coat each cap with egg mixture, drain briefly, then coat with crumbs, shaking off excess. Set caps, gill sides down, slightly apart on a 12- by 15-inch baking sheet. Lay bun halves, cut sides up, side by side on another 12- by 15-inch baking sheet.
Bake mushroom caps in a 450° oven until browned and flexible when pressed, about 20 minutes. About 3 minutes before mushrooms are done, add buns to oven; bake until lightly toasted.
In a small bowl, mix sour cream and chipotle chilies. Spread mixture on cut sides of buns. Layer equal amounts of onion and spinach on each bun bottom, top with a mushroom cap, add salt and pepper to taste, and cover with bun top.