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Preheat oven to 350 degrees Butter a 9 x 9-inch baking dish and set aside.
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In a large bowl, gently toss plums with sugar, corn starch, sea salt, and lemon zest and juice, and set aside.
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If making your own oat flour, grind oats on high in the bowl of a food processor or in batches in a coffee grinder until mixture is relatively smooth and flour-like. In a medium bowl, combine oat flour, cornmeal, sugar, and sea salt. Add butter, and rub into oat mixture with fingertips until mixture has a sandy, clumpy texture and is well combined.
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Spoon plum filling into prepared baking dish. Top with oat mixture. Bake 40 - 45 minutes, or until crisp topping is golden and filling is bubbling vigorously.
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Serve, warm or at room temperature, with vanilla ice cream.