Butternut Squash Casserole - Full Belly Farm

Ingredients

• 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