Preheat the oven to 425 edegrees with a rack in the center. greassse a 9x13 inch baking dish with the butter.
heat 2 tbsps of the olive oil in a large skillet over medium heat. once the oil is glistening add the leeks and ocok stirring often until softened about 4 min add the garlic and cook stirring until fragrant about 1 min more. Transfrer a large bowl add the shredded brussels sprouts to the bowl and toss to combine
Add the cream gruyere 1/4 cup of the parmesan salt pepper thyme and nutmeg to the same skillet bring to as immer stirring constantly until cheese melts and a thick sauce forms about 3 min. pour the sauce over brussells sprouts mixture and toss to coat.
Transfer the brussels sprouts mixture to the prepared baking dish bake until the brussels sprouts are tender about 20 min.
in a small bowl, combine the panko with the remaining 1/4 cup of parmesan and reamaining 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 min more.
serve family-style.
