Reminiscent of a fruit crisp, this sweet-tart dessert upends the usual proportions of jammy filling and buttery oatmeal crumble to great effect.
Preheat oven to 375 degrees. Butter a 9-by-13-inch baking dish. Simmer apples, cherries, and cider in a covered saucepan, stirring occasionally, until fruits are softened and plump, about 5 minutes. Drain, reserving 2 tablespoons liquid. Let cool.
Coarsely chop fruit. Combine with 2 tablespoons sugar, 1/4 teaspoon cinnamon, and reserved cooking liquid.
Whisk together oats, flour, baking soda, salt, remaining 3/4 cup sugar, and remaining 3/4 teaspoon cinnamon in a large bowl. Cut in butter with a pastry blender or 2 knives until crumbs range in size from peas to gum balls. Press 4 cups of oat mixture into bottom of prepared dish. Spread fruit filling on top, leaving a 1/4-inch border on all sides. Sprinkle remaining oat mixture on top.
Bake until golden brown, about 35 minutes. Let cool in dish on a wire rack. Cut into 24 bars.
Bars can be stored in an airtight container for up to 3 days.