Preheat the oven to 350ºF and grease a 9-inch pie dish, baking dish, or large cast-iron skillet (10-inch would be ideal).
Mix the apple slices, fresh cranberries, brown sugar, ground cinnamon, starch, lemon zest, and lemon juice in a large bowl until well combined. Transfer the fruit mixture to the prepared pan and spread it into a somewhat even layer.
In a separate bowl, combine the dry ingredients: oats, flour, brown sugar, cinnamon, and salt. Mix until combined, then work in the cubed butter with your hands until you're left with pea-sized clumps of crumble.
Sprinkle the crumble topping over the top of the fruit layer. Bake uncovered for 45 minutes. Allow it to cool down for a few minutes before digging in. Serve with a scoop of vanilla ice cream.