Steam the cauliflower over a pan of lightly salted, boiling water for about 12 minutes until tender. Drain and put in an ovenproof dish.
Meanwhile, melt the butter in a heavy-based saucepan, stir in the flour and cook for 1 minute. Slowly stir in the milk, then two-thirds of the Cheddar and heat, stirring constantly, until the sauce has thickened. Season with mustard, salt and pepper.
Pour the sauce over the cauliflower, sprinkle with the remaining cheese and scatter the breadcrumbs over the top. Put under a medium grill until the top is golden brown. Garnish with bacon strips.