Heat the oven: Preheat to 375°F (190°C). Grease a 9x9-inch baking dish or similar 2-quart dish.

Mix the filling: In a large bowl, combine blueberries, granulated sugar, cornstarch, lemon zest, lemon juice, vanilla, and salt. Toss until the berries are evenly coated and glossy.

Make the topping: In another bowl, stir together oats, flour, brown sugar, cinnamon, and a pinch of salt. Pour in the melted butter and mix with a fork until crumbles form. If using nuts or coconut, fold them in now.

Assemble: Spread the blueberry mixture in the baking dish. Sprinkle the oat topping evenly over the fruit, breaking up any large clumps for even browning.

Bake: Place the dish on a lined baking sheet to catch drips. Bake for 35–45 minutes, until the topping is deep golden and the berries are bubbling thickly around the edges. Frozen berries may take an extra 5–10 minutes.

Rest before serving: Let the crisp cool for at least 15–20 minutes. This helps the juices set so each scoop holds together.

Serve: Spoon warm crisp into bowls and top with ice cream or whipped cream. A little extra lemon zest over the top is a nice touch.