Preheat the oven to 375 degrees. Cut the squash in half lengthwise and place halves, flesh side down, in a baking dish with enough hot water to come 1/4-inch up the sides of the squash.
(The water prevents burning and helps steam the squash so it cooks more evenly.) Cover with aluminum foil and bake until the squash is soft when poked with a knife, 1 to 1 1/2 hours. Discard the seeds.
Scrape along the pulp with a fork and pull out and reserve shreds. Rub the bottom of a.
medium (8-cup or slightly larger) gratin or baking dish with butter.
Toss the "spaghetti" with the chopped sage, garlic, 2 cups of the cheese, cream, salt and pepper.
Spread mixture in gratin dish (the squash layer should be about 1 inch thick). Sprinkle the remaining cheese over "spaghetti." Arrange the 3 remaining sage leaves on top of the gratin. Bake for about 45 minutes.
Sprinkle freshly ground black pepper over gratin and serve.
Note: The squash can be baked a day ahead, following the above directions. If the top of the gratin doesn't turn golden brown, place it under the broiler for about 1 minute.