Preheat and prep: Heat your oven to 375°F (190°C). Lightly butter a 9-inch square baking dish or a similar 2-quart dish.

Mix the berry filling: In a large bowl, combine the berries, sugar, cornstarch, lemon juice, lemon zest, vanilla, and a pinch of salt. Gently toss until the berries are evenly coated. Spread the mixture into the prepared baking dish.

Make the almond topping: In a separate bowl, stir together the oats, flour, brown sugar, cinnamon, and salt. Add the sliced almonds and mix to distribute.

Cut in the butter: Add the cold butter cubes to the dry mixture. Use a pastry cutter or your fingertips to rub the butter into the dry ingredients until you get rough, pea-sized crumbs. Drizzle in the almond extract and toss to combine. The mixture should clump when pressed.

Assemble: Scatter the topping evenly over the berry filling, leaving a few small gaps for steam to escape.

Bake: Place the dish on a rimmed baking sheet to catch drips. Bake for 35–45 minutes, until the topping is golden brown and the filling is bubbling thickly around the edges. If using frozen berries, add 5–10 minutes.

Rest before serving: Let the crumble sit for at least 15–20 minutes so the juices thicken. Serve warm with ice cream, whipped cream, or yogurt.