Heat some oil in a large pan and cook the onion, garlic and potato for 3 minutes.
Add the stock, mustard and salt and pepper and simmer for 2 minutes.
Add the milk and the broccoli and cook for 10 minutes.
Blend until smooth, then stir in the grated cheese.
Pour into bowls and top with more grated cheddar (optional).
Video
Notes
You could add some cauliflower too to use a mixture of vegetables.
Prefer a stronger flavour? Use Stilton instead of mature cheddar.
If you are making this for young kids, mix some little bits of pasta in to it, to make more of a pasta soup stew.
You could make this vegan by using almond/soya milk and vegan cheese.
Try using shallots instead of onions for a sweeter flavour.
Top with some toasted seeds to add some crunch.
Give this a green spinach boost by adding some spinach to it. It will make it a really vibrant green colour too.
Use a stick blender to blend this soup. If you are going to use a jug blender, make sure it cools down first, as putting hot soup in can cause it to explode.