Preheat Oven: Heat oven to 350°F (175°C). Grease a muffin tin or line with paper liners.
Prepare Berry Filling: Mix berries with maple syrup, lemon juice, and cornstarch in a bowl. Make sure all berries are coated.
Make Crumble Topping: In another bowl, combine almond flour, oats, sugar, salt, and cinnamon. Cut in cold butter with a pastry cutter or fingers until mixture looks like coarse crumbs.
Assemble Cups: Fill each muffin liner about two-thirds with berries. Sprinkle crumble topping evenly over each cup.
Bake: Bake 25–30 minutes until topping is golden and berries are bubbling.
Cool & Serve: Let cool slightly before serving. Optional: top with vanilla ice cream or whipped cream.