Panna Cotta with Stewed Grapes

Ingredients

• 2 tablespoons cold water
• 1 3/4 teaspoons granulated unflavored gelatin
• 1 1/2 cups heavy cream
• 1/2 cup milk
• 1/4 cup sugar
• Pinch of salt
• 1 pound sweet harvest grapes, plus some halved grapes, for garnish
• 1/4 cup sugar, or more, if needed