01 - Preheat oven to 350°F. Lightly grease an 8×8-inch baking dish with butter or nonstick spray.
02 - In a large bowl, gently toss the mixed berries with granulated sugar, cornstarch, lemon zest, and 1 teaspoon vanilla extract until evenly coated. Transfer the mixture to the prepared baking dish and spread into an even layer.
03 - In a separate bowl, combine the rolled oats, almond flour, sliced almonds, brown sugar, salt, and cinnamon. Stir until well blended.
04 - In a small bowl, whisk together the melted butter, egg, and 2 teaspoons vanilla extract until smooth. Pour over the dry topping mixture and fold gently until just combined and crumbly.
05 - Sprinkle the almond topping evenly over the berry filling, covering the surface as uniformly as possible.
06 - Bake for 35 to 40 minutes, or until the topping is deeply golden and the berry juices are bubbling around the edges of the dish.
07 - Let cool for at least 10 minutes before serving. Serve warm with a dollop of yogurt, whipped cream, or vanilla ice cream if desired.