Scrape the zest off the lemon and squeeze the juice. You can either use a large lemon or two small ones
Put the butter and sugar in the mixer and beat for about 3 minutes, until they are well incorporated
Add the oats, flour, baking powder and salt and beat in slow speed
The mixture should look like breadcrumbs
Line a medium tin with some baking parchment
Put only half of the mixture in the tin and press it with a wooden spoon to form the base
Set the oven to 175 degrees
In a bowl put the zest, lemon juice and milk
Mix gently with a spoon until the juice is incorporated into the milk
Pour the milk mixture on the base and spread it evenly
Sprinkle on top the remaining crumbs and put it in the oven for 25'
Take it out and let it rest for about half an hour, then put it in the refrigerator for one more hour
Cut in pieces and enjoy!