Preheat oven to 350°F (175°C). Line an 8x8-inch square baking pan with parchment paper, leaving a 1-inch overhang on two sides to remove the bars easily. Lightly grease the parchment.

Make crust mixture: In a medium bowl, whisk together 1.75 cups all-purpose flour, 0.75 cup rolled oats, 0.5 cup packed light brown sugar, 0.25 cup granulated sugar, 1 tsp baking powder, and 0.5 tsp fine salt until evenly combined.

Cut in butter: Cut 0.75 cup (1.5 sticks) cold unsalted butter into small cubes and use a pastry cutter or fork to cut into the dry mixture until it resembles coarse crumbs with some pea-sized pieces remaining. Alternatively pulse briefly in a food processor.

Press into pan: Reserve about 3/4 cup of this crumb mixture for the topping (set aside in refrigerator). Press the remaining mixture firmly and evenly into the bottom of the prepared pan to form the crust. Use the bottom of a measuring cup to compact it. Bake for 12–14 minutes, until just set and starting to turn light golden at the edges. Remove from oven and set aside while you make the filling. Leave oven at 350°F (175°C).

Garden-Inspired Filling

Prepare vegetables and fruit: Finely grate or chop 0.75 cup peeled carrots and 0.75 cup zucchini. If zucchini is very moist, squeeze out excess liquid in a clean towel or paper towel and pat dry. Finely chop 0.5 cup peeled apple into small dice. Measure out 0.25 cup raisins or currants and 0.5 cup toasted chopped nuts.

Melt butter and combine sugars: In a medium saucepan or in a microwave-safe bowl, melt 0.5 cup unsalted butter with 0.5 cup packed light brown sugar and 0.25 cup granulated sugar, stirring until sugars dissolve and mixture is warm but not boiling. Remove from heat and let cool 2 minutes.

Finish filling batter: Whisk 1 large egg and 1 tsp vanilla into the warm butter-sugar mixture until smooth. In a separate bowl, whisk together 1 cup all-purpose flour, 1 tsp ground cinnamon, 0.5 tsp ground ginger, and 0.25 tsp fine salt. Stir the dry ingredients into the butter-sugar-egg mixture until just combined. Fold in the grated carrots, squeezed zucchini, diced apple, 0.25 cup raisins, and 0.5 cup toasted nuts so the filling is evenly distributed.

Assemble: Spread the filling evenly over the warm prebaked crust using an offset spatula or back of a spoon. Sprinkle the reserved 3/4 cup crumb mixture evenly over the filling, pressing lightly so it adheres but leaving some texture.

Bake: Return the pan to the oven and bake for 25–30 minutes, until the top is golden brown and the filling is set (a toothpick inserted into the center should come out with a few moist crumbs, not raw batter). If the top is browning too quickly, tent loosely with foil for the last 8–10 minutes.

Cool: Remove from oven and place on a wire rack. Allow the bars to cool completely in the pan (about 1–1.5 hours) so they firm up for clean slicing. Use the parchment overhang to lift the slab from the pan and transfer to a cutting board.

Finishing & Optional Toppings

Slice and finish: Dust the top lightly with 2 tablespoons powdered sugar and 1 tsp lemon zest if using. For extra shine and flavor, drizzle 1 tablespoon honey or maple syrup over the bars. Cut into 12 squares or 9 larger bars using a sharp knife wiped between cuts for clean edges.

Storage: Store cooled bars in an airtight container at room temperature for up to 3 days, or refrigerate up to 7 days. For longer storage, freeze individually wrapped bars for up to 3 months; thaw overnight in the refrigerator.