01 - Set oven to 350°F (180°C) and allow it to fully preheat.
02 - In a large mixing bowl, gently toss the mixed berries with granulated sugar, cornstarch, vanilla extract, and lemon zest until evenly combined.
03 - Spread the berry mixture evenly into a 9-inch (23 cm) baking dish.
04 - In a separate bowl, combine flour, oats, sliced almonds, light brown sugar, ground cinnamon, and salt. Add diced cold unsalted butter and vanilla extract. Work the mixture with a pastry cutter or fork (or rub between fingertips) until it forms coarse crumbs.
05 - Distribute the crumble topping evenly over the fruit mixture.
06 - Bake in the preheated oven for 35 minutes, or until the topping is golden and the fruit is bubbling around the edges.
07 - Allow to cool for 10 minutes before serving. Serve warm, optionally with vanilla ice cream or whipped cream.