How to Make It
Preheat oven to 425°F. Place whole grapes in a single layer on a rimmed baking sheet. Sprinkle evenly with sugar, and drizzle with 1/3 cup water. Bake in preheated oven until grapes begin to burst and sugar has dissolved, 15 to 20 minutes, stirring once after 10 minutes.
Transfer grape mixture and juices on baking sheet to a food processor; pulse until finely chopped and juicy, about 10 times. Using the back of a spoon, press mixture through a medium wire-mesh strainer into a bowl to yield about 1 1/3 cups grape juice mixture; discard solids. Stir in remaining 1/3 cup water.
Stir together grape juice mixture, tequila, lemon juice, and salt in a measuring cup with a spout. If using, place 1 tablespoon grape slices into each of 6 (3.3-ounces) paleta molds. Divide grape juice mixture evenly among molds. Cover with lid, and insert craft sticks. Freeze until firm, at least 8 hours.