Preheat the oven to 425°F with a rack in the center
Grease a 9x13-inch baking dish with the butter.Heat 2 tablespoons of the olive oil in a large skillet over medium heat
Once the oil is glistening, add the leeks and cook, stirring often, until softened, about 4 minutes
Add the garlic and cook, stirring, until fragrant, about 1 minute more
Transfer a large bowl
Add the shredded Brussels sprouts to the bowl and toss to combine.Add the cream, Gruyere, ¼ cup of the parmesan, salt, pepper, thyme, and nutmeg to the same skillet
Bring to a simmer, stirring constantly, until cheese melts and a thick sauce forms, about 3 minutes
Pour the sauce over Brussels sprout mixture and toss to coat.Transfer the Brussels sprouts mixture to the prepared baking dish
Bake until the Brussels sprouts are tender, about 20 minutes.In a small bowl, combine the panko with the remaining ¼ cup of parmesan and remaining 3 tablespoons of olive oil.Remove the Brussels sprouts from the oven
Evenly sprinkle with the panko mixture, then bake until the topping is golden brown, 5 to 6 minutes more
Serve family-style.