If you don't have a cherry pitter (I don't), then there are several work around ways in pitting the cherries. You can use a funnel, chopstick, icing tip, or a straw. I personally use an oversized straw.
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Place the fresh cherries into a skillet with cornstarch and sugar. Cook over medium heat, stirring occasionally until the cherries break down.
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Prepare the oat mixture in a bowl.
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Sprinkle the mixture over the top of the cherries and bake them for about 25-30 minutes.