Butternut Squash Gratin

I took this to a potluck, and didn't remember to photograph it until it was already in the bag. Note: This is not my recipe. It is Jacques Pepin's. But it is so good, it deserves repetition (with slight tweaking).

Peel a 3.25 lb butternut squash. Slice lengthwise, remove seeds (roast them separately for a tasty treat!) and slice squash into 1/4-1/8" slices. Put slices into saucepan. Cover with cold water and bring to a boil (this will probably take about 10-15 minutes). Boil 2 minutes; drain. Pour squash into gratin dish and cover with 1T kosher salt, freshly ground pepper, and 1 1/2 c heavy cream. Stir gently to combine. Sprinkle 4 T parmesan on top. Bake 30 min at 400 degrees. Brown 4 minutes under broiler before serving. Serve with fried sage as garnish.