INGREDIENTS
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4 portobello mushroom caps, cleaned
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1/4 cup olive oil
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Salt and freshly ground black pepper
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1/2 cup mayonnaise
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1 tablespoon prepared pesto
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2 teaspoons coarse-grained mustard
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4 hamburger buns, split, buttered, and toasted
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1 cup baby arugula
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1 (12-ounce) jar roasted red peppers, drained