Preheat oven to 375°F Butter a shallow, 2-quart baking dish.
In a mixing bowl, combine sugar, flour, blueberries and lemon juice; toss together. Transfer to baking dish.
In work bowl of a food processor, pulse hazelnuts until coarsely chopped. Remove half of hazelnuts to a mixing bowl. Add flour, sugar, brown sugar and salt to food processor. Pulse to combine. Add butter and pulse until mixture resembles course crumbs.
Add mixture to bowl with hazelnuts. Work together with hands until mixture clumps together. Crumble chunks of topping over blueberries.
Bake until filling bubbles and topping is golden brown and crisp, about 45-50 minutes.