Prep: Preheat the oven to 350F and line a 9x9 inch baking pan with parchment paper and spray lightly with cooking spray; set aside.
Make the Base & Crumble: Mix the flour, oats, almond flour, sugar, and baking powder together in a large bowl. Add the melted butter and vanilla and mix with a spatula, until a crumble forms. Take about 2 cups (240 g) of the mixture and use your hands to press it evenly into the bottom of the prepared baking dish.Mix the sliced almonds into the remaining “base” to form the crumble, then set aside.
First Bake: Bake the base in the preheated oven for 12 minutes, then set aside on a wire rack.
Jammy Filling: Mix the cornstarch with 2 tablespoons of cold water in a small bowl and set off to the side. Add the cherries and sugar to a nonstick sauté pan or medium pot over medium heat. Once the cherries have thawed I like to use a potato masher or fork to break them up more for a jammy filling, but you can leave them whole if you want your filling to be more chunky. Bring to a simmer; give the cornstarch slurry one more mix, then add to the cherries. Simmer for an additional 3 to 5 minutes, stirring frequently; the filling is done when you can drag a spatula through it and it leaves a clear path. Remove from the heat and stir in the lemon juice and almond extract, then pour the warm filling over the cooked crust, using your spatula to spread it out evenly.
Second Bake: Sprinkle the reserved almond crumble evenly over the cherry filling. Bake for 30 to 35 minutes, until the filling is bubbly and the top is golden brown.
Cool: Remove from the oven and set the entire pan on a wire rack to cool completely. Once the crumble is cool to the touch (about an hour) you can place the baking dish in the fridge to speed this up - mine took an additional hour to cool completely once refrigerated
Slice & Serve: Sprinkle the powdered sugar lightly over the bars. Use the parchment paper to remove the bars from the baking dish and transfer to a cutting board, then use a sharp knife to slice into 16 even bars. Store leftovers in an airtight container at room temperature for up to 2 days, or the fridge for up to 5 days. You can also freeze leftovers and reheat in the toaster oven as needed!