Oil a baking dish that can snugly fit the squash halves. Place the squash, flesh side down, in the dish and pour in 2 cups of water. Bake until it is fork-tender, about 20 minutes.
Meanwhile, heat the butter or oil in a frying pan and sauté the onions until they turn translucent. Add the celery, walnuts, sunflower seeds, and garlic. Cook over low heat until the nuts are browned. Add the sage, thyme, and marjoram. Stir in the breadcrumbs, lemon juice, salt, pepper, and raisins; then cook over low heat for 5 to 8 minutes. Remove from the pan and stir in the cheese.
Lower the oven temperature to 350°F. Pack the stuffing into the squash cavities and cover with aluminum foil. Bake, flesh side up, for 25 minutes. Then uncover and cook for 5 to 8 minutes longer to brown.