Add mushrooms, onion, garlic, and sherry to lightly greased skillet. Cook, covered, over medium heat until mushrooms are wilted, about 5 minutes. Continue cooking until vegetables are very tender and all liquid is absorbed, 8 to 10 minutes. Cool.
Process mushroom mixture and Parmesan cheese in food processor until smooth. Season to taste with lemon juice, salt, and pepper. Refrigerate 2 to 3 hours for flavors to blend.
Serve in crock with bread or crackers (not included in nutritional data).