Preheat oven to 425F. Cut Squash in half the long way and scoop out seeds and pith. Place cut side up on a rimmed baking sheet and brush lightly with oil. Season liberally with salt and roast in oven for about 50 minutes, or until browned and tender. It is important to not over-cook the squash, so be sure to check every so often during the final 20 minutes of cooking. The squash should be tender and shred, but not fall apart. Remove from oven and allow to cool slightly.
Turn the squash over and peel the skin from the outside. Using two forks, gently shred the squash into long strands. In a large mixing bowl, combine cooked squash, heavy whipping cream, butter, sour cream, corn starch, chili flake and half of the cheese. Mix well to combine, adjusting seasoning as necessary with salt and pepper, and place mixture into a greased 8”x8” cake pan. Sprinkle the remaining cheese over the top and place back in 425F oven for 25-30 minutes, or until top is golden brown.
While gratin is baking, heat a small amount of oil in a small sauce pot over medium heat to about 350F. Carefully place sage leaves a few at a time into oil. Be careful, as they will sputter at first and having a lid handy is advised. Allow sputtering to subside, about 5-10 seconds, and remove lid. Continue frying, stirring occasionally until leaves turn dark green, about 20-30 seconds total. Carefully remove and place on a paper towel to drain. Sprinkle lightly with salt and reserve. Crumble over gratin just before serving.