Cut the parsnips, sweet potato, carrots and courgette into chunks and place in a roasting tin. Add the onions and pepper, drizzle with oil and roast in a preheated oven at 180˚C (fan assisted) for 25 to 30 minutes or until cooked.
When cooked, mix with the tomatoes, tomato purée, chilli powder and herbs.
Meanwhile prepare the sauce by placing the butter and flour into a saucepan. Add the skimmed milk gradually, constantly stirring over a moderate heat. Bring to the boil.
Cook for 2 to 3 minutes until smooth and thickened. Add half the cheese and the mustard.
Assemble the lasagne by placing half the vegetables in an ovenproof dish, cover with three sheets of lasagne and top with half the sauce. Repeat layers ending with cheese sauce.
Sprinkle with remaining cheese and return to oven for 35 to 40 minutes until golden brown.