Yam and Winter Squash Casserole

Ingredients
3 large sweet potatoes 
2 pounds butternut, acorn or Hubbard squash, cut in half, seeds and strings removed
1 teaspoon olive oil
2/3 Cup unsalted butter, melted
4 eggs
1/3 Cup milk
1 teaspoon vanilla extract
Topping
1/4 Cup unsalted butter, melted
2/3 Cup (packed) light or dark brown sugar
1/3 Cup coarsely chopped walnuts
3 Tablespoons unbleached all-purpose flour
1/2 teaspoon ground nutmeg
1/2 teaspoon ground cinnamon
Preparation

1. Position a rack in the middle of the oven and preheat to 425 degrees.
2. Poke the yams in several places with a fork, and place them on a rimmed baking sheet lined with foil.
3. Brush the cut surfaces of the squash with olive oil. Place the squash, cut side down on the same baking sheet and bake until the squash is soft and tender, about 35 to 45 minutes.
4. Remove the squash from the baking sheet and set it aside. Return the baking sheet to the oven and continue baking the yams until they are very soft, another 10 to 15 minutes. Let the squash and yams cool to room temperature.
5. Peel the yams, discarding the skins and place them in a large bowl. Mash them roughly with a potato masher or an immersion blender, and measure out 2 Cups of the mash. Reserve the remainder for another use.
6. Peel the squash, discarding the skin, and place in a large bowl. Mash roughly with a potato masher or an immersion blender, and measure out 2 Cups of the mash. Reserve the remaining for another use.
7. Combine the mashed yams and squash in a large bowl and puree with a potato masher or an immersion blender until smooth. Add the melted butter, eggs, milk, and vanilla, and beat with a whisk or a wooden spoon until blended. Transfer the mixture to an 8-inch square baking dish.
8. Preheat oven to 350 degrees.
9. Prepare the topping: Place the melted butter, brown sugar, walnuts, flour, nutmeg, and cinnamon in a small blow, and stir to blend. Sprinkle the topping over the squash mixture.
10. Cover the baking dish with foil and bake for 30 minutes. Then remove the foil and continue baking until the topping if golden, about 20 minutes more. Serve hot.
Serves 6-8