How to Make It
Stir together flour, hazelnuts, oats, 1/4 cup brown sugar, and 1 teaspoon salt in a medium bowl. Add 1/2 cup butter. Pinch and smear butter between your fingers until butter is evenly incorporated and mixture resembles small pebbles. Cover and chill until ready to use.
Preheat oven to 400°F. Heat remaining 3/4 cup brown sugar in a 10-inch cast-iron skillet over medium. Cook, stirring occasionally with a wooden spoon, until melted and smooth, 8 to 10 minutes. Stir in apples, lemon juice, cinnamon, vanilla bean seeds, remaining 1/4 cup butter, and remaining 1/2 teaspoon salt until well combined. (Caramel will seize up initially, but then re-melt. The skillet will be very full, but after the apples cook down a bit, it will be easier to stir.) Cook, stirring often, until apples are just fork-tender and liquid is syrupy, 12 to 15 minutes.
Pour brandy into a heatproof measuring cup with a spout. Remove skillet from heat, turn off burner, and add brandy. Using a stick lighter, carefully ignite fumes just above surface of brandy. Carefully shake skillet until flames extinguish. Stir to incorporate residual brandy into apple mixture.
Sprinkle hazelnut mixture evenly over apples. Bake in preheated oven until topping is golden brown and filling is bubbly, 20 to 25 minutes. Let cool at least 20 minutes before serving. Serve crisp warm or at room temperature with crème fraîche.