Heat your oven to 200 degrees.
Heat a heavy based pan over a medium to high heat and spritz with low calorie cooking spray. Add the onion and fry for 5 minutes or until softened. Add the mince and fry for a further 5 minutes, breaking up the meat with a spatula.
Add the carrot and peas, and fry for a further 5 minutes.
Add the stock and the Worcestershire sauce, and stir well. Season to taste.
Bring to the boil, and then gently simmer for around 15 minutes, or until the liquid has reduced and reached your desired consistency. Stir occasionally to make sure that it doesn't stick to the bottom of the pan.
Meanwhile, boil your potatoes until they're soft (this will take around 10 minutes). Remove from the heat and mash.
Spoon your mince mixture into a large, shallow, ovenproof dish, or divide between 4 individual serving ovenproof dishes. Allow to cool slightly, and then spoon your mashed potato over the top, using a fork to fluff it up and add texture.
If using, sprinkle over your cheese.
Bake in the oven for 20 minutes, or until golden brown and piping hot.
Serve and enjoy!