Preheat the oven to 350 degrees F. Place 6 creme brulee dishes on a sheet pan lined with parchment paper.
In a large bowl, toss together the blueberries, raspberries, strawberries, 1/2 cup granulated sugar, the cornstarch, lemon zest, and lemon juice. Divide the mixture evenly among the creme brulee dishes, including any juices that collect.
For the crumble, combine the flour, granulated sugar, brown sugar, oats, cinnamon, and salt in the bowl of an electric mixer fitted with the paddle attachment. Add the butter and mix it on low speed until the mixture is crumbly. Pinch it with your fingers until it makes large crumbles and distribute it on the berries (it will not cover them entirely). Bake for 35 to 40 minutes, until the juices are bubbly and the topping is browned. Serve warm with a small scoop of vanilla ice cream.