Hi MG

Fry one roughly chopped onion in a little olive oil in a large heavy based saucepan - big enough to hold your whole soup.
When transparent add a head of cauliflower cut into florets and enough chicken stock to just cover.
Boil until tender.
Drain cauliflower/onion mix and reserve the chicken stock.
Bung cauliflower/onion in food processor and blitz til smooth - add a bit of chicken stock if it is too thick.
Back in your original saucepan, out about a tablespoon of butter/or marg and the same amount of flour. Make a roux. When it looks like buttery breadcrumbs, slowly add the reserved chicken stock back to the roux, stirring (I actually use a whisk) to get rid of any lumps. Add back the cauliflower puree and add a tin of light evaporated milk, or plain milk, or half milk-half cream if you want to be really decadent. Check the seasoning. I sometimes add curry powder, or nutmeg, or paprika, or even some freshly grated parmesan at this stage to jazz it up. Parsley also goes well. You can also add walnuts (at the boiling cauli stage) and blue cheese (at the final stage if you aren't pregnant of course!).

I use this method to make cream of any thing soup. It hasn't failed yet.