Preheat your oven to 180°C (350°F). Line your baking pan with parchment paper, leaving overhang on the sides for easy removal.
In a mixing bowl, combine the oats, flour, brown sugar, and a pinch of salt. Mix them together until evenly distributed.
Melt the butter in a saucepan over low heat until it’s smooth and fragrant, then pour it into the dry ingredients. Stir until the mixture becomes crumbly and holds together when pressed.
Press about two-thirds of this oat mixture firmly into the prepared pan, creating an even crust. Use the back of a spoon or your fingers to press it down smoothly.
In a small bowl, toss the cranberries with sugar and lemon zest until coated. Spread this tart, juicy mixture evenly over the crust.
Sprinkle the remaining oat crumb mixture over the cranberries, pressing lightly to help it adhere. This forms a crumbly topping.
Bake the bars in the preheated oven for about 25-30 minutes, until the top is golden and bubbling around the edges. The cranberries should be soft and fragrant.
Remove the pan from the oven and let the bars cool on a wire rack for at least 20 minutes. This allows the layers to set and makes slicing easier.
Lift the bars out of the pan using the parchment overhang. Slice into squares or rectangles with a sharp knife or pizza cutter.
Serve the bars slightly warm or at room temperature, enjoying the crisp edges and the burst of tart cranberries in each bite.
