Preheat the oven to 350 degrees F.
For the fruit filling:
In a large mixing bowl, toss together the apples, lemon juice, sugar, and flour. Pour the apple mixture into a buttered 2-quart baking dish and set aside.
For the topping:
In a large mixing bowl, mix the flour, rolled oats, brown sugar, cinnamon, and salt. With a food processor, a pastry blender, or your fingers work the butter into the flour mixture just until it comes together and large clumps form. Fold nuts into mixture.
Sprinkle the topping evenly over the fruit. Bake the apple crisp until the fruit is bubbling and the topping is golden brown and crisp, about 45 minutes.
Serve the crisps warm with vanilla bean ice cream or fresh whipped cream, if desired.