Butternut Squash Gratin with cinnamon

1 (2 -3 lb) butternut squash 2 tablespoons unsalted butter 3 cups halved and sliced onions salt 1/4 teaspoon black pepper 3 cups peeled peeled cored sliced[...]

3 (12 ounce) boxes frozen squash, thawed & drained (I use Birds Eye) or 3 lbs butternut squash, peeled,seeded & cut in cubes 6 tablespoons brown sugar 2[...]