Vinaigrette: In a very small saucepan boil sherry until reduced by about half and let cool. In a bowl whisk together sherry reduction, balsamic vinegar, garlic, shallot, sugar, salt and pepper to taste. Add oils in a stream, whisking until emulsified. Whisk in herbs. Reserve 1/4 cup vinaigrette for dressing mesclun. Transfer remaining vinaigrette to a large sealable plastic bag. Discard stems from mushrooms. Add mushrooms to bag and seal, pressing out excess air. Put bag in bowl. Marinate mushrooms, chilled, turning bag once or twice at least 15 minutes and up to 2 hours.
Remove mushrooms from bag, letting excess vinaigrette drip off. Grill on hot grill for about 5 minutes on each side or until done.
In a bowl toss mesclun with reserved vinaigrette and salt and pepper to taste and divide among 4 plates. Transfer mushrooms to cutting board and slice into 1-inch slices. Transfer sliced mushrooms to plates and garnish with with curls of ricotta salata.