Preheat oven to 350 degrees F.

In a medium bowl, add the crust ingredients (melted butter, sugar, and flour), and mix to combine. Press the crust evenly into a lightly sprayed 9 x 13 inch pan. I have this one - Amazon, Glass Baking Dish

Bake the crust for 15-20 minutes in the oven until firm and golden - you don't want much color on this since you will be baking it again with the filling and crumble, so no more than 15-20 minutes total for this bake.

Meanwhile, you can use the same bowl again to mix the crumble topping. Mix all ingredients until combined and set aside.

Using a different bowl, mix ingredients for the lemon filling. Add all ingredients and mix until combined.

Real lemons are the best for this, but if you don't have that, you can substitute 1/4 cup of lemon juice, as this is about what you get from 2 lemons.

When the crust is done in the oven, remove the pan carefully. Pour the lemon filling mixture on top of the crust, and evenly distribute the crumble on top. The crumble will fall into the mixture, as the lemon filling isn't thick, but that's okay! It will all bake together and that's what you want.

Next, put the pan bake in the oven, and bake for another 20 minutes. The bars will firm up as they cool, so remove after 20 minutes and let cool completely on a wire rack. Don't try to remove the bars from the pan until it has cooled completely, or it will fall apart.

You can top these with some powdered sugar, whipped cream, or enjoy as they are!