Preheat oven to 180C / 170C fan / gas 4 and line a 23 x 30cm deep sided oven tray.
In a large mixing bowl sieve the flour, salt and baking powder. Mix the oats in to the bowl and set aside.
In a free standing mixer with the paddle attachment (or using a hand whisk) cream the butter and sugar together for around five minutes until lighter in colour and fluffy.
Mix in the flour and oat mixture until is resembles a breadcrumb / crumble mixture. Take half the mixture and press it into the prepared tin using the back of a spoon, smooth it over and set aside.
To make the filling, whisk together the condensed milk, lemon zest and lemon juice until slightly thickened. Scrape the mixture onto the base and then sprinkle the remaining crumble mixture evenly over the top.
Bake in the preheated oven for 25-30 minutes until the top is turning golden and the filling is set. Take out of the oven and allow to cool completely. Refrigerate for at least 2 hours, preferably overnight and then slice into 16-20 squares. It is better to slice the lemon crunch when completely cold as then the filling does not ooze.
Keep in an airtight container on waxed paper in the fridge. See how long they last!
These also freeze very well, make double and freeze half and they will last for a month or so in the freezer.
Add 1/2 tsp of ground ginger or cinnamon to the topping for a different flavour combination.