Using food processor, process spinach, onion tops and parsley until fine. Mix with celery, melted butter and lemon juice. Fill 6 pie plates or tins half full with rock salt. Place 6 oysters on half shell in each plate. Cover each oyster with 1 teaspoon vegetable mixture. Place plates under broiler; cook until edges of oysters curl, then serve immediately. Makes 6 servings or 36 appetizers.
