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

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

Step 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

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

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.

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