4 tablespoons (1/2 stick) salted butter, at room temperature
1/2 cup lightly packed light brown sugar
Pinch of kosher salt
1 cup chopped pecans, toasted
6 Granny Smith apples
Juice of 1/2 lemon
Preheat the oven to 350°F.
Center the Brie in a pie plate. Set aside.
In a stand mixer fitted with the paddle attachment (or in a large bowl with a handheld electric mixer), beat together the butter, brown sugar, salt, and 2 tablespoons of water on medium speed until the mixture is paste-like. Add the pecans and beat just until combined. Spread the mixture on top of the Brie; it’ll be roughly 1 inch thick.
Bake until the Brie looks like its sides are about to give way, 10 to 15 minutes.
Meanwhile, core and slice the apples into wedges. In a large bowl, toss the wedges with the lemon juice to prevent browning.
Serve the warm Brie immediately with the apple slices.