Sauté the onion in the butter until softened but not coloured (about 5 minutes).
Add the potato, pour over the stock or water, cover and bring to the boil and then cook for 10 minutes.
Add the broccoli florets and cook for 3 minutes. Then add the peas and cook for a further 3 minutes.
It’s a good idea to puree this in a mouli or baby food grinder to get rid of the husks from the peas and also pureeing potato in a blender breaks down the natural starches and makes it sticky.