1. Preheat your oven to 375 F (190 C). Lightly grease an 8×8 inch baking pan or line it with parchment paper, leaving an overhang on the sides to easily lift the bars out later.

2. In a large mixing bowl, combine the unfed sourdough starter, unsweetened applesauce, maple syrup, and vanilla extract. Whisk these ingredients together until they are well blended and smooth.

3. In a separate medium bowl, whisk together the whole wheat flour, all-purpose flour, baking soda, and salt. Ensure there are no clumps of baking soda or salt.

4. Gradually add the dry ingredient mixture to the wet ingredients, stirring gently with a spatula or wooden spoon until just combined. Do not overmix.

5. In a small bowl, gently toss the diced fresh strawberries with the optional lemon juice and cornstarch or arrowroot powder.

6. Pour about two-thirds of the batter into the prepared baking pan and spread it evenly with your spatula. Distribute the prepared strawberries over this layer of batter. Then, dollop the remaining batter over the strawberries, spreading it as best you can to cover them. If using, sprinkle the rolled oats and turbinado sugar evenly over the top.

7. Place the baking pan in the preheated oven and bake for 20-25 minutes, or until the edges are golden brown and a toothpick inserted into the center of the bar comes out clean or with moist crumbs, not wet batter. The strawberries should be soft and bubbly.

8. Once baked, remove the pan from the oven and let the bars cool completely in the pan on a wire rack. Once cool, use the parchment paper overhang to lift the entire batch out of the pan and cut into desired bar sizes.