Adjust oven racks to upper-middle and lowest positions (you’ll use both at different times) and preheat oven to 400 degrees.

Start the fruit filling: Toss apples, 2 tablespoons brown sugar, lemon juice, ½ teaspoon salt, and ¾ teaspoon cinnamon together in a large bowl. Transfer to an 8-inch square baking pan or cast iron skillet with at least 2-inch sides and press into an even layer. Cover the pan with a lid or tightly with aluminum foil and place on a rimmed baking sheet. Transfer sheet pan to the oven and bake on the lower rack for 35 minutes.

While apples bake, prepare crumble topping: in a medium bowl, whisk together flour, ½ cup brown sugar, baking powder, 1 teaspoon cinnamon, and salt. Add butter and vanilla and mash with a fork or crumble together with your fingertips until moist clumps form and no dry flour remains. You can also pulse this in a food processor. I like the topping pieces to be about the size of peas or smaller.

Remove the sheet pan from the oven. Check to make sure the apples have started to get tender and shrink down a bit. Carefully stir in blackberries. Smooth the top of the apples with a spatula. Sprinkle the crumble topping evenly over the fruit. Transfer the sheet pan to the upper rack and bake until the topping is nice and golden and the filling is bubbling along the edges of the baking dish, 25 to 35 minutes. I always make sure my fruit desserts are bubbling before I take them out otherwise you may have a watery dessert. Place the baking dish on a wire rack and allow to sit at room temp for at least 45 minutes before serving. During this time, the juices will thicken up nicely. Serve with vanilla ice cream, if desired.