In a mixing bowl, whisk together brown sugar, flour, oats, cinnamon, and sea salt; add cubed butter and work it into the flour mixture with a pastry blender (or your hands) until mixture is crumbly. Cover and refrigerate until ready to use.
In an oil-sprayed, 9 x 7 baking dish, spread out sliced apples; add lemon juice and cinnamon; toss to coat.
Sprinkle the crumble mixture over the surface of fruit; Bake for 30-40 minutes at 350 degrees until topping is browned and fruit mixture is bubbling.