Heat the oven to 400F/200C/Gas Mark 6-7. Grease well a 2 1/2 pt (1.5l) ovenproof dish.
Break the cauliflower into florets, and slice any extra vegetables you are using. Chop the bacon, peel and slice the onion and fry them together in the oil over a moderate heat for 4-5 minutes, until the bacon is cooked. Mix in the flour or cornflour and gradually stir in the milk, cooking it gently until the sauce thickens. Mix in 3/4 of the cheese and season to taste.
Put the cauliflower and any extra vegetables into the greased dish. Pour the hot sauce over, mix the remaining cheese with the breadcrumbs, and scatter over the top of the sauce. Bake in the hot oven for 10-15 minutes, until crisp and golden brown on top.