Preheat the oven to 375°F. Lightly butter an 8-inch square baking dish or another 2-quart baking dish. Place it on a rimmed baking sheet to catch any bubbling cherry juices.
In a large mixing bowl, combine the pitted cherries, sliced plums, chopped prunes, sugar, cornstarch, cherry liqueur, fresh lemon juice, vanilla extract, almond extract, cinnamon, and salt.
Stir until the cherry mixture is evenly coated, then transfer the cherry filling to the prepared baking dish.
In a medium bowl, whisk together the gluten-free oats, almond flour, light brown sugar, cinnamon, and salt.
Add the cold butter. Use a pastry blender or your fingertips to cut the butter into the dry ingredients until the mixture resembles coarse crumbs with a few larger pieces.
Fold in the sliced almonds.
Scatter the crumble topping evenly over the cherry filling. Gently squeeze a few handfuls together as you go to create larger, crumbly clusters.
Bake for 40 to 50 minutes, until the topping is golden brown and the cherry filling is bubbling around the edges and through the center.
If the topping begins to brown too quickly, loosely cover the baking dish with aluminum foil for the remaining baking time.
Let the Cherry Crumble cool at room temperature for at least 15 minutes before serving. This gives the filling time to thicken.
Serve warm with a scoop of vanilla ice cream, whipped cream, crème fraîche, or Greek yogurt.