Heat the olive oil in a large skillet over medium heat. Once the oil is glistening, add the shallot and cook until softened, but not quite browned, about 3 minutes. Add the garlic and cook, stirring, until fragrant, about 1 minute more.
Add the butter to the skillet. Once the butter is melted, stir in the heavy cream, mustard, salt, pepper, garlic powder, nutmeg and bring to a simmer. Stir in the Brussels sprouts and reduce the heat to medium-low. Cover and cook until the Brussels sprouts are bright green and begin to wilt, about 8 minutes.
Meanwhile, heat a medium skillet over medium heat. Add the hazelnuts and cook, stirring occasionally, until fragrant and lightly toasted, about 5-8 minutes.
Stir the Parmesan into the Brussels sprouts. Cook, stirring, until thickened and creamy, about 5 minutes more.
Top the Brussels sprouts with the toasted hazelnuts and serve immediately.
