This is the easy way... I love a good cheese sauce to go with veges... and sometimes on it's own on toast!
Place a the butter into a saucepan and allow to melt. Add cornflour and use a whisk to combine. Keep whisking until you get a smooth paste.
Add milk - start with about 1/4c and combine well. Ensure you have a smooth consistancy after each addtion. If it isn't smoothing out, add a little more milk and keep whisking!
Once you have added all the milk, it should be the consistancy of thickened cream. Add the parmesan cheese and whisk until well combined.
Add cheddar to taste (more if you like it thivk, less if you like it runny). Season to taste.
* If you "white sauce" isn't getting smooth, there's a couple of things you can try... 1. Add more milk/stock/water and keep whisking 2. Take it off the heat - it cooks the mixture quickly and you don;t have a chance to get rid of the lumps 3. if all else fails, don't throw it away, use a sieve