1. Preheat oven to 350°F (175°C) and line an 8×8-inch pan with parchment paper.

2. In a bowl mix oats, flour, brown sugar, and salt. Stir in melted butter until mixture forms coarse crumbs.

3. Press two thirds of the oat mixture firmly into the bottom of the prepared pan to form the crust.

4. In a separate bowl toss blueberries with granulated sugar, cornstarch, lemon juice, and vanilla until evenly coated.

5. Spread the blueberry filling over the crust in an even layer.

6. Sprinkle the remaining oat mixture over the blueberries, adding nuts if using.

7. Bake for 35–40 minutes until the topping is golden and berries are bubbling.

8. Cool completely on a wire rack, then refrigerate 30 minutes for clean slices.

9. Lift bars from pan using parchment overhang and cut into 16 squares.

10. Serve at room temperature or chilled, optionally with ice cream.