Toast the bread crumbs in a jelly-roll pan in the middle of a preheated 350° for 3 to 5 minutes, or until they are golden, and let them cool.
In a saucepan, cook the onion in 3 tablespoons of the butter over moderately low heat, stirring, until just melted. Stir in the flour and cook the roux over low heat, stirring, for 3 minutes.
Remove the pan from the heat and add the milk, scalded, in a stream, whisking vigorously until the mixture is thick and smooth. Simmer the sauce, stirring occasionally, for ten minutes, add the cheddar cheese, and cook the sauce, stirring, until the cheese is melted. Season the sauce with salt and the white pepper and let it cool. Cover its surface with plastic wrap.
In a pot of boiling salted water, cook the florets for five minutes, or less, until the florets are just tender. Drain the cauliflower, plunge it into a bowl of ice and cold water to stop the cooking, and drain it well.
Pat the cauliflower dry with several thick paper towels, and transfer to a buttered 2-quart oven-proof baking dish.
In a small bowl, toss the bread crumbs with the remaining 1 tablespoon butter, melted, pour the cheddar sauce over the cauliflower, and sprinkle the cauliflower with the bread crumbs.
Brown the cauliflower with sauce and bread crumbs under a preheated broiler about 4 inches from the heat.
Please Note: Nutritional values listed may vary from final results depending on actual measurements and brands used to prepare the recipe.