Prepare your batter by first mixing together the sugars and butter in a large mixing bowl. Use a hand mixer for this, and spend about 2 minutes mixing with the mixer on the lowest speed. Since the butter is melted, we just need to spend a bit of time making sure the liquified butter gets completely incorporated!
Add the egg and vanilla to the wet ingredients and mix to combine thoroughly. I use the hand mixer on the lowest setting and spend about 15 seconds combining at this stage.
Add the flour, salt, baking powder, baking soda, cinnamon and nutmeg to the batter and mix until you can't see any more bits of white flour.
Add the oats and use a wooden spoon or rubber spatula to combine.
Add the chopped white chocolate and stir to combine.
Working quickly, form the dough into golfball sized balls and pop them on a parchment-lined baking sheet. The dough is a very wet dough, and it's also warm from the melted butter. So, it might look like some of your cookies are splooshing out a bit as they rest on the baking sheet, before you chill them. Don't panic! Just work fast and get them in the fridge to chill, they'll be fine once they've firmed up in the cold! Chill the dough balls in the fridge for at least half an hour, up to overnight.
When you're ready to bake, preheat your oven to 170FAN/350F. Arrange the cookie dough balls on parchment lined baking trays, allowing for about an inch of space around each cookie.
Bake for 13-14 minutes, until the cookies are brown on top. Allow cookies to cool on a wire rack. Enjoy!