Cook the potatoes in boiling, salted water until just tender. Drain.
Heat the oil in a frying pan and fry the onions for 2-3 minutes. Add the mushrooms, peppers and sweetcorns and cook for 2-3 minutes until tender. Stir in Schwartz for Chef Herbs, Spices and tomato purée.
Meanwhile, melt the butter in a small pan, add the flour and cook for 1 minute. Gradually add the milk, stirring. Bring to the boil, stirring constantly until thickened.
Add half of the cheese. Mix together the potatoes, vegetables and sauce and transfer to a casserole dish. Top with remaining cheese and bake for 30 minutes.