Position oven rack in the middle and preheat to 350°F (175°C), then line a 9×13 inch (23×33cm) light-colored aluminum baking pan with two overlapping sheets of parchment paper leaving at least 2 inches of overhang on all sides.

In a large mixing bowl, whisk together the all-purpose flour, rolled oats, brown sugar, granulated sugar, and fine sea salt until evenly combined — about 15 seconds.

Scatter the cold butter cubes over the dry mixture and cut in using a pastry blender or two forks, working quickly for 3 to 4 minutes until the largest butter pieces are pea-sized and the mixture holds together when squeezed but falls apart when released.

For Option A (scratch filling): combine pitted cherries, granulated sugar, cornstarch, and lemon juice in a medium saucepan, stir until the cornstarch dissolves, then cook over medium heat stirring constantly for 6 to 9 minutes until the filling thickens, turns glossy dark ruby, and bubbles actively at the center — remove from heat, stir in vanilla, and set aside to cool for 5 to 10 minutes.

Measure out two-thirds of the oat mixture (approximately 3 heaped cups) and scatter evenly across the prepared pan, then press firmly and evenly into a compact ¼-inch (6mm) layer using the flat bottom of a measuring cup, making sure to press all the way to the edges and corners.

Bake the crust alone at 350°F (175°C) for 12 to 15 minutes until the surface looks matte and dry and the very edges have just begun to turn faintly golden, then remove from the oven without turning it off.

Pour the cherry filling (scratch-made or canned) over the warm pre-baked crust and spread evenly to the edges using a silicone spatula, then scatter the remaining one-third of the oat crumble mixture loosely over the filling in irregular clumps without pressing down.

Return the assembled pan to the oven and bake at 350°F (175°C) for 28 to 32 minutes until the crumble peaks are light to medium golden, the bottom crust edges are deep golden, and the cherry filling is actively bubbling at the center of the pan — not just at the edges.

Transfer the pan to a wire cooling rack and cool at room temperature for the full 2 hours without moving to the refrigerator — the cherry filling continues to firm through cooling and cutting earlier will cause the filling to collapse.

After the 2-hour cool, whisk together the sifted powdered sugar, 2 tablespoons of whole milk, and vanilla extract until smooth, adding the remaining tablespoon of milk if needed to reach a thick drizzle consistency, then transfer to a piping bag or zip-lock bag with a small corner snipped and drizzle diagonally over the uncut slab.

Allow the glaze to set for 10 minutes, then use the parchment overhang to lift the slab onto a cutting board and cut into 24 bars using a sharp knife wiped clean between every cut — 4 cuts across the width and 6 cuts along the length.