In 1-qt enamel, glass or stainless stell pan, bring water and lemon juice to a boil. Add sliced mushrooms; cover pan. Reduce heat and simmer gently for 2 - 3 minutes. Remove from heat. Drain in sieve; pat dry with paper towels.
In 1-qt bowl, combine cream, onion, sugar salt and pepper. Add mushrooms. Toss lightly until well covered. Serve as a salad on crisp, dry lettuce.