Preheat the oven to 350°F.
Grease a 9x13-inch baking dish with butter or nonstick cooking spray.
Working with 2 sheets of phyllo dough at a time, fold the sheets into accordion-style folds.
Place the folded phyllo dough into the prepared baking dish.
Repeat the process until all phyllo dough has been used.
Gently separate and fluff the folds to ensure they are evenly spaced throughout the dish.
Bake the folded phyllo dough for 10 minutes.
Remove the baking dish from the oven.
Drizzle the melted butter evenly over the top of the baked phyllo.
Return the dish to the oven.
Bake for an additional 10 minutes.
While the buttered phyllo bakes, prepare the custard mixture.
In a medium bowl, whisk together the milk, eggs, sugar, dried thyme, garlic powder, and salt until fully combined.
Remove the baking dish from the oven.
Insert the thinly sliced apples and brie cheese evenly into the folds of the phyllo dough.
Pour the milk and egg mixture evenly over the phyllo, apples, and brie.
Return the baking dish to the oven.
Bake for 30 to 35 minutes, or until the top is golden brown and the custard is set.
Remove the dish from the oven.
Drizzle the warm bake with honey.
Sprinkle the chopped walnuts evenly over the top.
Allow the bake to cool slightly before slicing.
Serve warm or at room temperature.