Preheat the oven to 400°. Cut the squash in half crosswise and scoop out the pulp and seeds. Trim the ends so each half will stand upright. Place both halves in a baking pan, flesh side up, and pour hot water into the pan to reach about halfway up the squash. Add 1/2 tablespoon butter to the center of each squash and season with salt and pepper. Loosely cover the pan with foil. Bake the squash until tender when pierced with a fork, 45 minutes to 1 hour.
Prepare the couscous according to the package directions and set aside. Heat the olive oil in a large skillet over medium-high heat. Add the onion and cook, stirring, until softened, about 4 minutes. Push the onion aside, add the meat, season with salt and pepper and cook through, about 5 minutes. Stir in the walnuts, cranberries and couscous; season with salt and pepper.
Remove the squash from the oven and, using a spatula, carefully transfer each half to a plate. Pour out any excess liquid from the centers. Divide the couscous among the squash.