1-First Step: Preheat your oven to 350°F (175°C) and line a 9-inch square baking pan with parchment paper, leaving some overhang on the sides. This makes it simple to lift the bars out later and ensures they don’t stick.

2-Second Step: For the crust, whisk together 1 and 1/2 cups all-purpose flour, 1 cup of the old-fashioned whole rolled oats, 1/2 cup packed brown sugar, 1 teaspoon baking powder, 1 teaspoon lemon zest, 1/4 teaspoon ground cinnamon, and 1/8 teaspoon salt in a large bowl. Pour in 10 tablespoons of melted unsalted butter and stir until the mixture turns into moist, crumbly sand with no dry spots.

3-Third Step: Press about two-thirds of this crumb mixture firmly into the bottom of your prepared pan to form the crust. Pop it in the oven and bake for 10 minutes, then take it out and let it cool slightly while you move on.

4-Fourth Step: In a saucepan over medium heat, combine 4 and 1/2 cups fresh blueberries, 1/2 cup granulated sugar, 2 tablespoons cornstarch, and 1 tablespoon lemon juice. Stir gently and cook for 2-3 minutes until everything dissolves and the berries are evenly coated. Remove from heat and mix in 2 teaspoons lemon zest.

5-Fifth Step: Pour the blueberry filling evenly over the warm crust in the pan. Take the remaining crumb mixture, stir in the reserved 2 tablespoons of oats, and sprinkle it over the filling. Press it down lightly for an even layer.

6-Sixth Step: Bake the whole thing for 45-55 minutes, until the top is lightly browned and the filling bubbles at the edges. Keep an eye on it to avoid over-baking.

7-Final Step: Pull the pan from the oven and let it cool completely on a wire rack for at least 2 hours. For easier cutting, pop it in the fridge once cooled. Lift the bars out using the parchment overhang and slice into squares. Serve them up warm with ice cream or chilled for a quick treat and enjoy the gooey goodness.