Preheat the oven to 200°C / 400°F / Gas mark 6.
Cut any large sprouts in half and toss them with the oil and some seasoning in a roasting tin or ovenproof dish.
Roast for 20 minutes until the sprouts are tender and turning golden.
Meanwhile, cook the bacon lardons until crisp and remove leaving the fats and oils in the pan.
Add the breadcrumbs and crisp up in the bacon fat until golden and crunchy.
Remove from the pan to cool, then wipe the pan clean with kitchen paper.
Melt the butter in the pan with 2 tablespoons of milk, then stir in the flour and mustard powder for 2 minutes which will make a paste.
Gradually whisk in remaining milk, until there is a smooth sauce.
Stir and cook until the sauce comes to the boil and thickens.
Turn off the heat and stir in the cheese until melted. Taste and season if necessary.
Tip the sprouts into a baking dish, scatter with the crisp bacon lardons, then spoon over the sauce and scatter with the crispy crumbs.
Bake the gratin for 10 minutes (or 20 minutes from chilled) at 200°C / 400°F / Gas mark 6 until piping hot and bubbling.
Can be cooled, covered and chilled for up to 48 hour before baking.