Meanwhile, melt the margarine in a small saucepan, add the flour and combine thoroughly. Cook gently for 1 minute, stirring. Add the mustard and stir until it is mixed through, then pour in the soy milk gradually, whisking continuously until it thickens and forms a smooth sauce. Bring to the boil and simmer gently for 4-5 minutes.

Put the cauliflower in a medium ovenproof dish, pour the sauce over the top and put the dish under a pre-heated grill for 5 minutes, until top is lightly golden.