Ricotta and Spinach Stuffed Butternut Squash

1 butternut squash (about 2 1/2 pounds)
1 10-ounce package frozen, chopped spinach, thawed and water squeezed out (see note)
1/2 cup ricotta cheese (I use whole milk ricotta)
1 egg
2 tablespoons grated Parmesan cheese plus extra for sprinkling on top, if desired
2 tablespoons grated or minced onion
2 tablespoons milk
1/2 teaspoon kosher salt and a few grinds of the pepper mill
1/4 teaspoon freshly grated nutmeg

Preheat oven to 400 degrees.
Cut squash in half lengthwise. Place cut sides down in a baking dish filled with about an inch of water. Bake 25-30 minutes or until barely tender. Remove from oven and pour off any remaining water.
While squash is baking, mix the remaining ingredients in a medium bowl. I think it is easiest to combine everything but the spinach and then work that in last.
Allow squash to cool for a few minutes. Then cut in half lengthwise again. Stuff the recessed area with the spinach mixture and bake for an additional 15 minutes or until the squash is tender and the topping is just starting to brown.
Remove from oven and sprinkle with additional Parmesan, if desired.

I think the easiest way to remove water from the thawed spinach is to wrap it in a clean tea towel and literally wring it out. You will be amazed out how much liquid you can quickly and easily remove this way.