First, preheat your oven to 180C/160C fan/Gas 4. Line a large baking tray with baking paper (non-stick greaseproof paper) – no need to stick it down.
In a large mixing bowl, mix together the caster sugar and butter with a wooden spoon until pale and fluffy – this should take a few minutes and your arm will be aching by the end if you forgot to take the butter out the fridge in preparation!
Add in the egg and beat to combine. Finally, add the oats, plain flour and cranberries and mix everything together until all is evenly distributed.
Cover your bowl with a bit of cling film and chill in the fridge. This step is important, whether you leave it 10 minutes in the fridge (minimum) or 3 days. Many say that the longer you leave it, the more lovely flavours and complexities develop; I’d say this is probably true.
Use a teaspoon or wet hands to dollop large plops of cookie dough onto your baking tray – leave plenty of space for them to expand out the way. Using big, heaped teaspoons, you’ll get between 12 and 18 cookies. Push each down a bit before baking.
Bake for 10-12 minutes or until a pale golden brown. If you fancy, you can sprinkle some brown sugar onto your dough balls before baking to give them a nice finish. Cool on their tray or a wire rack; it doesnae matter!
When buying your dried cranberries, check the ingredients. You want ones that are 100% cranberry – some will have lots of added sugar and oils to make them soft. Avoid these if possible.