01 - Preheat oven to 400°F. Lightly grease a 9x13-inch baking dish with butter or cooking 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. Drain thoroughly and set aside.
03 - In a large skillet over medium heat, melt the butter. Add the chopped onion and sauté for 3-4 minutes until translucent. Stir in the minced garlic and cook for 1 additional minute until fragrant.
04 - Pour the heavy cream into the skillet and bring to a gentle simmer. Stir in 3/4 cup Parmesan cheese, mozzarella, salt, pepper, thyme, and nutmeg. Continue cooking for 2-3 minutes, stirring constantly until the cheeses have fully melted and the sauce has thickened slightly.
05 - Fold the blanched Brussels sprouts into the creamy sauce until evenly coated. Transfer the entire mixture to the prepared baking dish, spreading it into an even layer.
06 - In a small bowl, combine the panko breadcrumbs, remaining 1/4 cup grated Parmesan, and olive oil. Toss until the breadcrumbs are evenly moistened, then sprinkle the mixture uniformly over the casserole.
07 - Bake for 20-25 minutes until the casserole is bubbly and the topping is golden brown. Allow to cool for 5 minutes before serving.