Add blueberries, lemon juice, 2 Tbsp flour, sugar, and a pinch of salt in a 9-inch pie dish or 2-quart casserole dish. Toss to combine
If you’re starting with frozen berries, microwave them for 1-1.5 minutes until they have enough moisture to hold all the flour and sugar so that it doesn’t pool at the bottom of the pan.
Combine oats, flour, sugar and salt in a medium bowl. Add melted butter and vanilla and stir until flour and oats are moistened.