Preheat the oven to 350˚. For the crumble: In a medium bowl, combine the oats, flour, brown sugar, cinnamon and salt. Stir in the melted butter until all of the dry bits are moistened and small clumps form. Place the bowl in the refrigerator for 30 minutes, or freezer for 10 minutes, while you make the filling. For the filling: In a large bowl, combine the sugar and corn starch. Stir in the blueberries, vanilla extract and lemon juice until well coated. Butter a three quart casserole dish. Place the blueberry filling in the casserole in an even layer and top evenly with the crumble. Bake for 45 minutes until the edges are bubbly and the topping is crisp and golden brown. Let rest for 15 minutes then serve with whipped cream or vanilla ice cream.