Spray a 6-quart slow cooker with non-stick cooking spray or line with parchment paper.
In a large bowl, toss together the sliced apples, cherries, 1/4 cup of the sweetener, lemon juice, cinnamon, and nutmeg until the fruit is well coated.
Pour the fruit mixture into the bottom of the prepared slow cooker and spread evenly.
In a medium bowl, combine the almond flour, chopped nuts, remaining 1/4 cup sweetener, and sea salt.
Add the melted butter and vanilla extract to the almond flour mixture and stir until the mixture becomes crumbly and resembles coarse sand.
Sprinkle the topping evenly over the fruit layer in the slow cooker.
Place a clean kitchen towel under the lid to absorb excess moisture, then cook on low for 4 hours or until the apples are tender and the topping is golden around the edges.
Let the crisp cool for 15 minutes before serving warm with optional sugar-free whipped cream or vanilla ice cream.