Preheat your oven to 350°F (175°C). Lightly butter or spray a 9x9-inch baking dish or similar 2–2½ quart casserole dish. A glass or ceramic dish works best for even baking.

In a large mixing bowl, combine the blackberries, 1/2 cup granulated sugar, lemon juice, lemon zest, cornstarch, and 1 teaspoon vanilla extract. Gently toss with a spatula or large spoon until the berries are evenly coated and the cornstarch is no longer visible. If the berries are very tart, taste one and add 1–2 tablespoons additional sugar if needed.

Pour the blackberry mixture into the prepared baking dish and spread it into an even layer, leaving a little space at the top to allow the fruit to bubble up as it bakes.

In a separate medium bowl, whisk together the rolled oats, all-purpose flour, brown sugar, granulated sugar, cinnamon, salt, and nuts (if using). Break up any clumps of brown sugar with your fingers or a fork.

Add the cold, cubed butter to the oat mixture. Using a pastry cutter, two knives, or your fingertips, work the butter into the dry ingredients until the mixture looks like coarse crumbs with some pea-sized pieces of butter remaining. If using, stir in 1 teaspoon vanilla extract at the end.

Sprinkle the oat topping evenly over the blackberry filling. Do not press it down firmly; leave it loose and nubbly so steam can escape and the topping can crisp.

Place the baking dish on a sheet pan to catch any drips. Bake at 350°F (175°C) for 40–50 minutes, or until the berries are bubbling vigorously around the edges and in the center, and the topping is deep golden brown and crisp. If the topping starts to brown too quickly before the filling bubbles, tent the dish loosely with foil for the last 10 minutes.

Transfer the baked crisp to a wire rack and let it rest for 15–20 minutes before serving. This allows the filling to thicken. Serve warm with vanilla ice cream, whipped cream, or thick Greek yogurt if desired.