Butternut Squash Gratin without oregano

1 tablespoon butter 1 tablespoon olive oil 2 large onions , chopped into 1-inch squares (about 4 cups) 3 tablespoons fresh sage , chopped or 1 tablespoon[...]

5 cups cubed peeled butternut squash (1/2-inch cubes) 1 teaspoon olive oil 1 1/2 cups thinly sliced leeks 2 cups cubed peeled braeburn apples (1/2-inch cubes)[...]

2 lbs butternut squash, peeled 1 1/4 lbs idaho potatoes , peeled (I used sweet potatoes) salt & freshly ground black pepper 6 tablespoons grated parmesan[...]