Preheat your oven to 350°F (175°C). This takes about 10–15 minutes. While it heats, lightly butter an 8x8-inch square baking dish or a 9-inch pie plate so the crisp doesn't stick to the pan.

In a large bowl, combine 5 cups fresh blueberries (or frozen, thawed and drained), 2 tbsp granulated white sugar, 1 ½ tbsp cornstarch, 1 tbsp fresh lemon juice, and ½ tsp fresh lemon zest.

Toss the blueberry mixture gently with a spatula for 1–2 minutes until the white cornstarch powder completely dissolves and coats all the berries evenly. The mixture should look slightly glossy and the berries should be well coated. Checkpoint: You are ready to move on when there are no visible white powder streaks on the fruit and the mixture looks uniform.

Pour the blueberry mixture directly into your prepared baking dish, spreading it in an even layer across the bottom.

In a separate bowl, whisk together ¾ cup old-fashioned rolled oats, ½ cup light brown sugar (packed), ½ cup all-purpose flour, ½ tsp ground cinnamon, and ¼ tsp fine sea salt for about 30 seconds until combined.

Take 6 tbsp unsalted butter (cold and cubed) and drop all the pieces into the oat mixture at once.

Using a pastry cutter, two forks, or your fingertips, work the cold butter into the dry ingredients for 2–3 minutes. Rub and pinch until the mixture resembles clumpy, textured gravel with pea-sized and marble-sized bits of butter still visible throughout. Do not overwork or the topping will become dense and greasy. Checkpoint: You are ready to move on when the mixture looks crumbly with visible butter chunks and no large dry patches remain.

Scatter the oat topping evenly across the surface of the blueberries in the baking dish. Do not press it down; leave it light, loose, and rustic-looking so the top stays crispy.

Place the baking dish on the middle oven rack and bake for 35–40 minutes at 350°F (175°C). Start checking at the 35-minute mark.

The topping is done when it turns a deep golden-brown color (not pale or tan). You should also see dark purple or burgundy blueberry syrup bubbling vigorously along the interior edges of the pan where the fruit meets the sides. If the topping is not golden yet, bake for another 2–3 minutes and check again. Checkpoint: You are ready to remove the crisp when the topping is golden-brown all over and juices bubble at the edges.

Remove the baking dish from the oven using oven mitts. Place it carefully on a wire rack to cool.

Let the crisp rest undisturbed for exactly 20 minutes at room temperature. This step is mandatory. During this time, the cornstarch continues to thicken the blueberry juices, transforming them from a thin, runny mixture into a thick, spoonable glaze. If you skip or rush this step, the filling will be too soupy to serve neatly.

After 20 minutes, the crisp is ready to serve. Divide into bowls or plates and top each serving with a scoop of vanilla ice cream while the crisp is still warm. The warmth will begin to melt the ice cream, creating a delicious, creamy sauce.