Cook the parsnips and leeks in boiling slightly salted water for 10 minutes until just tender. Drain well.
Cook the pasta according to the instructions on the packet and mix with the cooked vegetables.
Place milk, butter and flour in a saucepan and heat, whisking continuously until the sauce thickens and boils.
Season lightly with salt and pepper, add the Cayenne and mustard and cook for one minute. Remove from the heat and stir in 75g of cheese.
Place half the vegetable mixture in the base of a buttered ovenproof dish, cover with a layer of tomato then half the sauce. Repeat the layers, finishing with sauce.
Mix remaining cheese with the breadcrumbs and sprinkle over the top.