1. Prepare your baking dish and oven: Begin by preheating your oven to 375 F (190 C). Lightly grease an 8×8 inch square baking dish or line it with parchment paper, leaving an overhang on two sides to easily lift the bars out later. This ensures your bars won’t stick and makes for easy cleanup.

2. Combine the dry ingredients: In a large mixing bowl, whisk together the all-purpose flour, rolled oats, packed light brown sugar, baking soda, and salt. Make sure there are no lumps of brown sugar and that all the dry ingredients are evenly distributed. This forms the base of your oat mixture.

3. Mix in the wet ingredients: In a separate medium bowl, combine the melted unsalted butter, sourdough discard, and vanilla extract. Whisk these together until they are well combined and smooth. The sourdough discard should be at room temperature for the best incorporation.

4. Form the crumbly mixture: Pour the wet ingredient mixture into the dry ingredient mixture. Use a fork or your fingertips to mix until the ingredients are just combined and form a crumbly, coarse mixture. Be careful not to overmix; you want a texture that resembles wet sand with some larger clumps.

5. Assemble the bars: Press about two-thirds of the crumbly mixture evenly into the bottom of your prepared baking dish. Use the back of a spoon or your hands to compact it firmly. This will form the bottom crust of your breakfast bars.

6. Add the blueberries: Scatter the fresh or frozen blueberries evenly over the pressed oat mixture in the baking dish. If using frozen blueberries, there’s no need to thaw them first.

7. Top and bake: Crumble the remaining one-third of the oat mixture over the blueberries. Gently press it down lightly, but don’t compact it as much as the bottom layer. Place the baking dish in the preheated oven and bake for 25-30 minutes, or until the top is lightly golden brown and the edges are set. The blueberries will be soft and bubbly.

8. Cool completely before slicing: Once baked, remove the dish from the oven and let the bars cool completely on a wire rack before attempting to slice them. This is a crucial step; if you cut them while warm, they will be too crumbly. Cooling allows them to firm up and hold their shape beautifully.