Trim and chop the leeks before washing them thoroughly.
Peel the potatoes and chop/dice.
Dice the onion.
In a large pan on a low heat melt the butter and then add in the leeks, onion and potatoes. Mix everything together. I usually add a little extra butter on top to make sure everything is covered.
Season with salt and pepper and leave everything to sweat for 15 minutes on a low heat.
Add the stock and milk to the pan and leave for around 20 minutes. Check everything is soft to ensure it's ready.
Leave everything to cool and then blend. I use a hand blender to do this.
Try your soup and add any extra seasoning.
Gently reheat on the hob or in the microwave. Add chilli flakes (I do this per bowl as my toddler also enjoys this soups) and a slice of your favourite bread and enjoy.