Have a vegan guest coming to dinner? Use additional olive oil or coconut oil in place of the butter.
Rinse farro under cold running water. Combine farro and water in a medium saucepan; bring to a boil over high heat. Reduce heat to medium-low and simmer, stirring occasionally, until farro is tender, about 30 minutes. Stir in cherries after 27 minutes of cooking time. Drain cooked farro mixture.
Heat oil in a medium skillet over medium heat. Add onion, celery and carrot; cook, stirring often, until tender and beginning to brown, 10 to 12 minutes. Add sage; cook, stirring constantly, for 1 minute. Remove from heat and stir in farro mixture, beans, hazelnuts, ¾ teaspoon of the salt and ½ teaspoon of the pepper.
Cut each squash in half lengthwise; scoop out and discard seeds. Place squash cut-side up on a large rimmed baking sheet. (Trim bottoms, if necessary, so squash will sit flat.) Brush insides of squash with 2 tablespoons of the butter; sprinkle with remaining ¾ teaspoon salt and ½ teaspoon pepper. Bake until tender, about 45 minutes.
Spoon farro mixture evenly into squash halves, pressing lightly and mounding in the center. Drizzle farro with remaining 2 tablespoons butter. Return to oven and bake until filling is lightly browned, 10 to 15 minutes.