Place the onion and garlic into a pan with 50ml of the vegetable stock. Cover with a lid and allow to simmer for a few minutes until starting to soften.
Add the rest of the stock, broccoli, mustard powder and season with salt & pepper. Cover with a lid and allow to simmer for 20 - 25 minutes or until the broccoli is soft.
Remove from the heat and add the cheese triangles. With a food processor or hand blender, blend the ingredients until smooth.
Place over a low heat and stir in the yogurt and grated cheddar cheese. Stir for 1 - 2 minutes before serving.