Rinse 100 g (3½ oz/1 cup) rolled (porridge) oats in cold water, cover with just-boiled water and leave to soak for 20 minutes. Alternatively, soak overnight in cold water. Drain, rinse and transfer to a blender. Add 750 ml (26 fl oz/3 cups) cold water and blitz until smooth. To make really smooth oat milk, strain through a piece of muslin, a nut bag or a clean piece of cut-off tights and squeeze out the milk. Return the milk to a clean blender, blitz and strain again. Repeat if necessary for ultra-smooth milk.