Scoop out the squash seeds and roast them for a nutritious snack.
Preheat oven to 400 degrees. Cut squashes in half lengthwise. Scoop out seeds and reserve, if desired (see note, above).
Brush cut sides of squash halves with 1 teaspoon oil; season with salt and pepper. Place, cut sides down, on a rimmed baking sheet. Combine grapes, shallot, and sage in a bowl and drizzle with remaining 2 tablespoons oil. Season with salt and pepper; place around squash. Roast, stirring grape mixture once, until squash is tender and grape mixture is caramelized, 35 to 40 minutes. To serve, fill squash with grain and grape mixture.