Israeli couscous, which is a type of pasta, holds up well during the long turkey-roasting time, retaining its firm texture.
Preheat oven to 375 degrees. Drizzle squash with 1 tablespoon oil; season with salt and pepper. Place, cut sides down, on a rimmed baking sheet; roast 10 minutes.
Meanwhile, toss turnips and celery root with 2 tablespoons oil; season with salt and pepper. Place on baking sheet with squash. Continue roasting, stirring once or twice, until vegetables are tender and golden brown, about 30 minutes.
Meanwhile, heat butter and remaining tablespoon oil in a medium saucepan over medium heat until butter is melted. Add shallots; cook until softened, about 3 minutes. Stir in coriander, cumin, and paprika; cook until fragrant, about 1 minute. Stir in couscous, stock, bay leaf, 1 tablespoon thyme, and 1 teaspoon salt. Bring to a boil. Cover; reduce heat to low. Cook until couscous is tender but al dente, and liquid is absorbed, about 6 minutes. Remove from heat. Let stand, covered, 2 minutes. Fluff with a fork.
Peel squash; cut flesh into 1/2-inch cubes. Stir together couscous, vegetables, raisins, almonds, parsley, and remaining tablespoon thyme; season with salt and pepper. Serve, or immediately pack loosely in turkey cavity, and cook until an instant-read thermometer inserted into center of stuffing registers 165 degrees.