Trim the cauliflower and break into florets. Cook gently in boiling salted water for 5 mins, drain and set to one side.
Melt the butter in a large pan and add the flourcook for 1 to 2 mins. Stirring all the time then remove from the heat. Add the chicken stock and onions, return to the heat. Stirring constantly bring to the boil then simmer for 20 mins.