Butternut Squash Gratin without olive pomegranate mozzarella

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[...]

1/4 cup unsalted butter 4 cups thinly sliced onions (about 1 pound) 2 1/2 lbs butternut squash , peeled, seeded, cut into 1/2-inch cubes 1 teaspoon sugar 1/2[...]