In a small bowl, toss the blueberries with the sugar, flour, lemon juice, and salt until evenly coated.
Spoon the blueberry mixture into a 5×5-inch or 4×6-inch baking dish, or divide it between two 10-ounce ramekins.
In a second bowl, stir the flour, oats, and brown sugar together, then add the vanilla and softened butter and work them in with a fork until crumbly.
Scatter the topping evenly over the blueberries.
Bake for 25 to 30 minutes, until the topping is golden brown and the filling bubbles at the edges.
Let the crisp cool for 10 minutes, then serve warm, topped with vanilla ice cream or whipped cream if you like.
Notes
Soften the butter before you cut it in. Soft butter bakes into a crisp, crumbly topping, while melted butter soaks into the oats and bakes flat.Bake on a rimmed sheet pan. The filling can bubble over the sides, and a sheet pan catches the drips and makes the hot dish easier to lift out.Taste a few berries before you add the sugar. Blueberries swing from sweet to tart depending on the batch, so use a little less sugar for sweet berries and a little more for tart ones.Check the topping near the end. If it's browning faster than the filling is bubbling, lay a piece of foil loosely over the top so it doesn't over-brown before the fruit is done.If doubling this recipe, use a 6x8 inch baking dish.