Make the Filling: Peel the apples and cut them into 3/4- to 1-inch chunks.
Place in the bottom of a 6-quart slow cooker.
Add the maple syrup, cornstarch, lemon zest (I zest it right over the crockpot), lemon juice, cinnamon, ginger, nutmeg, and salt.
With a large spoon, stir to combine. Spread the apples into an even layer.
Make the Topping: In a medium bowl, stir together the oats, flour, brown sugar, granulated sugar, cinnamon, and salt.
Pour in the melted butter, then stir until evenly moistened.
Sprinkle over the top of the apples.
Place a clean kitchen towel or a double stack of paper towels on top of the slow cooker so that the sides drape over the long ends. Place the lid on top, using it to secure the towel(s) in place** (the towels absorb moisture, helping the topping better crisp up).
Cook the crockpot apple crisp on high for 2 1/2 to 3 1/2 hours or on low for 5 to 7 hours, until the apples are soft and cooked through when pierced with a fork. Remove the lid and towels. Serve hot with toppings of choice.
TO MAKE AHEAD: Prepare the topping as directed. Freeze it in a ziptop bag until you're ready to bake the crisp.
TO STORE: Refrigerate apple crisp in an airtight storage container for up to 4 days.
TO REHEAT: Gently rewarm leftovers in a baking dish in the oven at 350 degrees F or in the microwave.
TO FREEZE: Freeze apple crisp in an airtight freezer-safe storage container for up to 3 months. Let thaw overnight in the refrigerator before reheating.
**DISCLAIMER: Please note that using a towel with a slow cooker could be a fire hazard. Proceed at your own risk. I have done it many times over the years with different recipes and different slow cookers and have never had an issue. That said, different slow cookers can reach different temperatures. If this is your first time using the towel method with your slow cooker, I recommend checking the towel an hour into cooking to be sure that it isn’t too hot.