5cupssweet cherriesfresh or frozen; stemmed and pitted; see notes.
Prepare the cherry filling in advance as it needs to cool. Place cherry filling ingredients in medium sauce pan and stir well. Turn the heat to medium and bring to a boil. Reduce heat to low and simmer, stirring frequently, for 8-10 minutes. Remove from heat and let cool completely. If you want the filling to be smoother and less chunky, use a hand blender to slightly break the cherries into smaller pieces.
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.
Spread cherry pie filling over oats.
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.
If using sour cherries, bump the sugar by two tablespoons and lower the amount of lemon juice to two tablespoons. This will account for the natural tartness of sour cherries and balance it out.