Peel, core, and thinly slice 3 large apples (or 4 medium apples). We recommend an apple peeler that also cores and slices them for you. Then, cut the apple slices in half and add them to a large bowl.
Add in the blackberries and lemon juice. Gently mix with a spatula or wooden spoon.
Add the granulated sugar, flour, cinnamon, salt, and vanilla. Gently mix until the fruit is evenly coated and set aside. Allow it to rest for 5 minutes for natural fruit juices to release.
FOR THE CRUMBLE
Slice the butter into cubes and place it in the freezer. The butter needs to be very cold when you add it to the crumble mixture.
In a food processor, add the flour, brown sugar, cinnamon, allspice, cloves, and salt. Remove the butter from the freezer and add it to the food processor.
Use the pulse button on the food processor to mix the ingredients in short increments until it has a sandy consistency without any large butter chunks and no dry flour visible. Be sure not to overmix to avoid a pasty consistency.
Add the walnuts and rolled oats to the crumble and pulse again just until the walnuts are chopped into medium-sized pieces (4-6 short pulses should do the trick). Again, take care not to overmix the crumble.
ASSEMBLE & BAKE
Preheat the oven to 375°F and lightly grease a 10-inch glass or ceramic dish. Add the filling to the dish, spreading it evenly.
Sprinkle the crumble evenly over the top of the filling until the fruit is completely covered.
Bake on the middle rack for 35-40 minutes. The topping will be golden brown and the fruit will be bubbling. Let it cool for at least 15 minutes and serve with a side of ice cream, topped with whipped cream, or on its own.
Notes
Using an apple peeler/corer/slicer is the fastest and easiest way to prepare your apples for this recipe and get extra thin slices.
Cube and freeze the butter before preparing the other ingredients to ensure it's cold enough before adding it to your crumble.
You can omit the rolled oats and/or walnuts from the crumble if desired.
You can use frozen blackberries instead of fresh ones if needed.