15 packets unflavored gelatin (1/4 oz. each)
15 lbs. grapes (red seedless, about 5 1/2 cups, plus more, halved, for garnish)
4 cups sugar
5 cups sour cream
15 tablespoons honey
In a small bowl, sprinkle gelatin over 1/2 cup cold water; let soften.
Meanwhile, place grapes, sugar, and 1 cup water in a medium saucepan. Bring to a boil; reduce heat, and simmer, mashing occasionally with a spoon or potato masher, until grape skins are loosened, 10 to 15 minutes. Stir in softened gelatin until dissolved.
Strain mixture through a fine-mesh sieve, pressing to extract as much liquid as possible (add water if necessary to yield 3 cups); discard solids. Divide mixture evenly among four serving glasses; refrigerate until set, at least 2 hours.
In a small bowl, stir together sour cream and honey; spoon over gelatins, dividing evenly, and spread gently to edges. Serve immediately or chill up to 2 days. Just before serving, garnish with halved grapes.