01 - Preheat oven to 400°F. Lightly grease a 9x13-inch baking dish with butter or non-stick spray.
02 - Bring a large pot of salted water to a rolling boil. Add the trimmed and halved Brussels sprouts, blanching for 3-4 minutes until just tender-crisp. Drain thoroughly and set aside.
03 - In a large skillet over medium heat, melt the butter. Add the finely chopped onion and sauté for 3-4 minutes until translucent and fragrant. Stir in the minced garlic and cook for an additional minute.
04 - Pour the heavy cream into the skillet and bring to a gentle simmer. Add ¾ cup grated Parmesan, shredded mozzarella, salt, black pepper, dried thyme, and ground nutmeg. Stir continuously for 2-3 minutes until the cheeses have fully melted and the sauce thickens slightly.
05 - Fold the blanched Brussels sprouts into the creamy sauce, coating evenly. Transfer the entire mixture into the prepared baking dish, spreading into an even layer.
06 - In a small bowl, combine the panko breadcrumbs, ¼ cup grated Parmesan, and olive oil, tossing until the breadcrumbs are evenly moistened. Sprinkle the mixture uniformly over the casserole surface.
07 - Bake for 20-25 minutes until the edges are bubbling and the topping is golden brown. Allow the casserole to rest for 5 minutes before serving.