Fry mince and onion in a frying pan over medium high heat until browned. Add peas, carrots and beans; stir well. Pour in chopped tomatoes, tomato puree, Worcester sauce, salt and pepper. Simmer for 30 minutes, stirring occasionally.
Meanwhile, boil the potatoes till soft. Drain well, then mash, adding milk, butter, mustard powder, salt and pepper.
Place the mince mixture in an ovenproof dish and top with mash. Score mash and sprinkle with cheese if desired.
Bake in a preheated 190 C / Gas 5 oven on the middle shelf for 30 to 40 minutes, or until potato is golden brown.