Preheat your oven to 350°F (175°C).
In a mixing bowl, combine the fresh blueberries, granulated sugar, cornstarch, lemon juice, vanilla extract, ground cinnamon, and salt. Toss gently until the blueberries are evenly coated. Set aside.
In a separate bowl, make the crumble topping by mixing the softened butter, all-purpose flour, rolled oats, and brown sugar. Use a fork or your fingers to blend until the mixture resembles coarse crumbs.
Pour the blueberry mixture into the pie dish, spreading it evenly across the bottom.
Sprinkle the crumble topping generously over the blueberries.
Bake in the preheated oven for 30-40 minutes, or until the topping is golden brown and the blueberry filling is bubbly.
Allow to cool for at least 10 minutes before serving.
For an extra touch, serve warm with a scoop of vanilla ice cream or a dollop of whipped cream.