1. Start by peeling the butternut squash. Cut it in half lengthways and remove the seeds. Slice the squash with a mandolin as thinly as possible, or a knife if a mandolin is not available.
2. Halve, peel and thinly slice the onion.
3. Melt 150g of the butter in a large sauce pan over a low heat, being careful not to burn it. Add the onion and butternut squash and sweat over a low heat for 10 minutes.
4. Add the water and turn up the heat. Simmer for 30 minutes or until the vegetables are completely soft.
5. Remove from the heat and liquidize the soup. Pass through a fine sieve. Add the milk to obtain the right consistency - the soup should be silky-smooth. Be careful you do not add to much milk, which will dilute the flavour.
6. Finish by whisking in the rest of the butter and check the seasoning. Stir in a dash of cayenne pepper just before serving.
Recipe courtesy of Great British Chefs - www.greatbritishchefs.com.