Peel and chop the onion into small pieces. Peel the potatoes and chop them into small equal sized pieces.
Put the butter and oil into a saucepan and heat over a low heat to melt the butter. Add the chopped potatoes and onion to the pan and fry the vegetables for 3-4 minutes.
Add 500ml of the stock and heat until the stock boils. Put a lid on the pan and simmer for 8-10 minutes until the potatoes are soft.
Remove the pan from the heat and let it cool for 5 minutes. Meanwhile pull the tiny broccoli flowers from the heads, you don't need the stems. (The leftover broccoli stems can be used in a risotto).
Use a potato masher to carefully mash the cooked potato into the stock. (Be careful - it will still be hot).
Add the broccoli flowers and the rest of the stock to the pan and heat the soup until it boils. Reduce the heat (to simmer) and cook the soup for 3 minutes until the broccoli is just cooked. We want bright green broccoli!
