In medium skillet, melt 1 tablespoon of unsalted butter over medium heat. When the butter is melted, add the dijon mustard, minced shallot, pinch of freshly ground pepper and shaved Brussels sprouts; toss the mixture together and cook until softened, about 3 to 5 minutes. Salt to taste (my Dijon mustard was very salty so I added only a teeny pinch). Transfer to a small bowl and set aside.

Spread the remaining 1 tablespoon of unsalted butter on both pieces of bread. Flip the slices over (the buttered sides are the ones that are gonna hit the pan) and liberally add brie to one piece of bread. Top with the Brussels sprouts mixture and the other piece of bread. Carefully transfer to the skillet and cook on low until the bread is toasty and golden brown on both sides and cheese is gooey and melty, about 10 minutes.