Butternut Squash Casserole

Submitted to us by CSA member Heather MacClelland, this is a delicious way to use up that butternut squash!

3 1/2 pounds butternut squash, peeled and cut in 1/4-inch-thick slices (about 6 cups)
5 tablespoons unsalted butter, plus more for the pan
1 tablespoon extra-virgin olive oil
2 large yellow onions, trimmed, peeled and chopped (about 3 cups)
1 cup whole or low-fat buttermilk
2 large eggs, beaten
2 1/2 teaspoons minced fresh thyme
1/2 cup bread crumbs, toasted
3/4 teaspoon kosher salt
1/2 teaspoon freshly ground black pepper
1/4 cup pumpkin seeds or pecans, toasted
2 cups coarsely grated cheddar cheese (about 6 ounces)
Optional: 4 to 6 large handfuls of fresh spinach, washed and stemmed (add to the same water as the squash in step 2 during the last minute of boiling)

Preheat the oven to 375 degrees F. Butter a 9-x-13-inch baking dish or 4-quart casserole.
Place 3 quarts water in a large stockpot and bring to a boil over high heat. Add the squash to the water, return to a boil, and cook for 6 minutes. The squash will turn a deeper orange. Drain it and set aside.

Melt the butter in the stockpot and add the oil. Add the onions and sauté over medium-low heat until translucent and limp, about 8 minutes, stirring to prevent browning. Add the warm squash and the buttermilk, eggs, thyme, bread crumbs, salt, pepper, 1/8 cup pumpkin seeds and 1 1/3 cups cheese. Blend with a wooden spoon or spatula until the ingredients are well combined.

Spread the mixture evenly in the baking dish. Bake for 45 minutes. Remove from the oven and sprinkle with the remaining 1/8 cup pumpkin seeds and 2/3 cup cheese. Bake 15 minutes more. Remove from the oven and serve immediately.