Preheat oven to 350 degrees F. Line a baking sheet with parchment paper.

Make brown sugar walnut crust. In a food processor, pulse walnuts until finely ground. Add flour, brown sugar, salt, and butter, pulsing until mixture resembles coarse sand. Alternatively, chop walnuts by hand and combine with remaining ingredients using a pastry cutter. Slowly add water, one tablespoon at a time just until the dough comes together. Shape into a rectangle, wrap in plastic wrap and chill in the fridge for one hour or until firm.

Meanwhile, prepare cream cheese filling. Beat together softened cream cheese with egg yolk, caramel, and vanilla until well combined. Set aside.

Combine maple syrup with cinnamon, ginger, allspice, and nutmeg in a small pot. Bring to a simmer over medium heat and simmer for 1 minute. Remove from heat and let cool slightly.

Add spiced maple syrup to melted butter, vanilla, and salt, whisking until combined. Set aside half of the maple butter sauce, about ½ cup. Toss apples with remaining maple sauce, making sure every slice of apple is coated.

Lightly dust a clean work surface with flour. Roll brown sugar walnut crust until it's ¼-inch thick. Spread caramel cream cheese filling in the center, leaving a 1-inch border. Arrange apple slices on top, overlapping the apples to create a pattern. Fold over the edges to seal in the apples and brush the edges with egg wash.

Bake galette for 50 minutes - 1 hour or until golden brown and apples are softened. Remove from the oven and let cool slightly. Serve warm with vanilla ice cream if desired and extra maple butter sauce.