Preheat oven to 350. Arrange squash halves, cut side up, in two 9-by-13-inch baking dishes. Season with salt and pepper. Sprinkle sage over each, dividing evenly. Pour 1/2 cup stock into each baking dish and scatter garlic around squash. Cover with parchment, then foil, and bake until squash are tender when pierced with a fork. 45 to 55 min. Remove from oven.
Heat broiler with rack about 8 inches from heat source. Use a slotted spoon to transfer garlic to a small bowl. Mash with a fork, then stir in cream and 2 tablespoons liquid from baking dishes. Spoon evenly over squash halves, including edges. Broil until bubbling and golden,3 to 4 minutes. Serve immediately.