Yields Makes 4 servings
Notes: Buy portabella mushroom caps. Or buy whole mushrooms, and trim off stems and save them for other uses.

How to Make It

Step 1
1

In a bowl, beat egg whites and oil to blend. In another bowl, mix crumbs and chili powder.

Step 2
2

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.

Step 3
3

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.

Step 4
4

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.

Ingredients

 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
  Salt
  Pepper

Directions

Step 1
1

In a bowl, beat egg whites and oil to blend. In another bowl, mix crumbs and chili powder.

Step 2
2

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.

Step 3
3

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.

Step 4
4

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.

Crusted Portabella Burgers

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