Cook florets in lightly salted boiling water for 7 minutes until the stalks are just tender. Drain and keep warm.
To prepare sauce, melt butter and add flour to make a roux or paste. Gradually blend in milk using a balloon whisk, stirring through to boiling point for a couple of minutes. The sauce should be smooth and sufficiently thick to coat the back of a spoon.
Stir in parsley and drained pickled onions. Season to taste.
Preheat grill and place cauliflower in a heatproof dish. Pour over onion sauce and scatter with breadcrumbs. Season with pepper. Grill until crumbs are toasted then serve topped with the crispy bacon.
You can make this without bacon for a tasty vegetarian dish.