Prepare the cherry filling in advance as it needs to cool. In a medium size saucepan, bring the cherries, sugar, and lemon juice to a boil over medium heat. Reduce heat and simmer for 10 minutes, until the cherries become tender. Whisk in cornstarch and boil, stirring, until thickened, about 1 minute. Pour the filling into a bowl. If you want the filling to be smoother and less chunky, use a hand blender to slightly break the cherries into smaller pieces. Cover with a plastic wrap and let cool to room temperature. The filling may be made up to a day or two in advance and refrigerated.

Preheat the oven to 350F.

Line a 9 x 12 inch baking pan with parchment paper and set aside.

Next prepare the base and the top dry layers. In a large bowl, stir together the oats, flour, brown sugar, baking powder and salt. Cut in the butter to form a crumbly mixture.

Spread 2/3 of the oat mixture and pat firmly and evenly into the bottom of the baking pan.

Spread the cherry filling evenly over the base.

Crumble the rest of the oat mixture evenly over the cherry filling.

Bake at 350F until light golden brown, about 40 minutes. Transfer to a cooling rack and cool completely before cutting.