Peel all the vegetables and cut into bite-size pieces and boil for 10 minutes or until just tender. Drain and place in a bowl.
Preheat the oven to 180ºC / 350ºF / Gas mark 4.
Melt the butter in a saucepan and stir in the flour. Remove the pan from the heat and stir in the milk until there are no lumps. Bring to the boil, stirring until thickened and smooth. Add the mustard, thyme, seasoning and half the cheese.
Pour the sauce over the vegetables, stirring until coated. Spoon into a large ovenproof dish, sprinkle with the remaining cheese and breadcrumbs and bake for 20-25 minutes until crusty and golden on top.