1. Preheat the oven to 350°F (175°C) and lightly grease a 9×13-inch baking dish.

2. Squeeze as much moisture as possible from the thawed spinach, then spread it out in the prepared baking dish.

3. In a large skillet, melt the butter over medium heat, then add the chopped onion and cook until softened, 4 to 5 minutes; stir in the minced garlic and cook 30 seconds more.

4. Sprinkle the flour over the onion mixture and cook, stirring constantly, for 1 minute to remove the raw flour taste.

5. Slowly whisk in the milk until smooth, then simmer for 3 to 4 minutes, stirring often, until the sauce thickens enough to coat the back of a spoon.

6. Remove the skillet from the heat and whisk in the sour cream, salt, pepper, nutmeg, 1 cup of the Monterey Jack, and 1/4 cup of the Parmesan until the cheese melts.

7. Pour the creamy cheese sauce evenly over the spinach in the baking dish, gently stirring or nudging with a spatula so the sauce works down into the greens.

8. In a small bowl, combine the panko, remaining 1/2 cup Monterey Jack, remaining 1/4 cup Parmesan, and olive oil or melted butter, tossing until the crumbs are evenly coated.

9. Sprinkle the cheesy breadcrumb topping over the casserole and bake for 25 to 30 minutes, until the edges are bubbling and the top is golden brown.

10. Let the casserole rest for 5 to 10 minutes before serving so it can set slightly and scoop into neat squares.