4 tablespoons (1/2 stick) unsalted butter
2 large yellow onions, diced
¼ cup flour
¼ teaspoon nutmeg, freshly grated (or ½ teaspoon if using dried, ground nutmeg)
1 cup heavy cream
2 cups milk
3 pounds frozen chopped spinach, defrosted (5 10-ounce pkgs)
1 cup freshly grated parmesan cheese
1 tablespoon kosher salt
½ teaspoon freshly ground black pepper
½ cup grated white cheddar cheese
1 cup Japanese (Panko) breadcrumbs
½ cup dry grated parmesan
¼ cup pine nuts
1 teaspoon kosher salt
½ teaspoon ground black pepper
Preheat oven to 425 degrees.
Place all ingredients for the breadcrumb topping in a food processor and pulse just until coarsely ground; set aside.
In a large skillet or saute pan, cook the onions in the butter over medium heat until translucent, stirring often, about 15 minutes. Add the flour and nutmeg and for 2 more minutes, stirring constantly.
Add the cream and milk and cook until thickened, stirring occasionally, about 8 to 10 minutes. The sauce should be thick enough to heavily coat the back of a spoon.
Once thawed, chop the spinach and use your hands to squeeze as much liquid as possible from the spinach. Add spinach to the sauce with a ½ cup of the Parmesan cheese and stir to mix well. Season with salt and pepper; taste and adjust as desired.
Transfer the spinach to a baking dish and cover the top with remaining half-cup of Parmesan cheese and the cheddar cheese. Finish with breadcrumb coating as desired. If adding whole pine nuts on top, wait to add them until 5 minutes left in baking time, or they will burn as the casserole bakes.
Bake for 20 to 25 minutes until hot and bubbly. Serve immediately.