In a large mixing bowl, combine rolled oats, all-purpose flour, and sugar. Drizzle in melted unsalted butter, then mix thoroughly until the mixture resembles coarse crumbs.

Transfer the crumb mixture into a greased 9x9-inch baking pan, pressing it firmly into an even layer. Bake the base in the preheated oven for about 10 minutes, or until it’s slightly golden.

While the crust bakes, whisk together freshly squeezed lemon juice, lemon zest, sugar, and eggs in a separate bowl. Blend until the mixture is smooth and well-combined.

Once the crust is ready, carefully pour the lemon filling over the warm baked crust in the pan. Return to the oven and bake for an additional 20–25 minutes.

Remove the baking pan from the oven and allow the bars to cool in the pan for at least 30 minutes. Once completely cooled, slice into even bars.