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

2. Whisk together whole wheat flour, oats, baking powder, and sea salt in a large bowl

3. In a separate bowl, whisk sourdough starter, eggs, honey or maple syrup, oil, and vanilla until smooth

4. Pour wet ingredients into dry ingredients and stir until just combined

5. Fold in diced strawberries, lemon zest, and nuts or seeds if using

6. Spread batter evenly in prepared pan and press a few strawberry slices on top

7. Bake for 28–35 minutes until edges are golden and a toothpick comes out with moist crumbs

8. Cool in pan on a rack for 15 minutes, then lift out using parchment and cool completely

9. Use a serrated knife or bench scraper to cut into 9 bars

10. Store bars in an airtight container at room temperature for up to 2 days or refrigerate up to 5 days