01In a very small saucepan boil sherry until reduced by half and let cool. In a bowl whisk together reduced sherry, balsamic vinegar, garlic, shallot, sugar, and salt and pepper. Add oils in a slow, steady stream, whisking until emulsified. Whisk in herbs and reserve ¼ cup vinaigrette for dressing mesclun. Transfer remaining vinaigrette to a large sealable plastic bag.
02Discard stems from mushrooms. Add mushroom caps to bag with vinaigrette and seal, pressing out excess air. Put bag in a bowl. Marinate mushrooms, chilled, turning bag once or twice, at least 30 minutes and up to 2 hours.
03Place a rack in center of oven; preheat to 350°F.
04Melt butter in a large skillet over medium heat. Add onion and cook, stirring, until soft (about 5 minutes) and transfer to a bowl. Stir in prosciutto, bread crumbs, basil, parsley, and 1 cup freshly grated Parmesan. Season with salt and pepper to taste and mix until combined.
05Remove mushrooms from bag, letting excess vinaigrette drip off, and arrange mushrooms, gill sides up, in a 13x9x2-inch baking pan. Divide stuffing among mushrooms and press gently and evenly into caps. Pour water and sherry around mushrooms and bake 20 to 25 minutes, or until stuffing is golden brown.
06Toss mesclun with reserved vinaigrette in a medium bowl. Season with salt and pepper and divide among four plates. Transfer a mushroom to each plate and garnish with Parmesan curls.
Editor’s note: This recipe was first printed in the November 1998 issue of ‘Gourmet’ as Stuffed Marinated Portabella Mushrooms. Head this way for more of our best mushroom recipes →