In a large oven proof pan (I used cast iron) heat the oil over a medium heat and sweat the onions under a lid until tender, stirring occasionally (about 8 minutes).
Push the onions to the outside, turn the heat up a little, add the mince and fry until browned all over.
Add another tbsp of oil then add all the chopped vegetables and fry for a couple of minutes stirring.
Next add the flour and cook for a further minute, stirring.
Preheat oven to 180ºC.
Add the beef stock, Worcestershire sauce, horseradish sauce, seasoning and peas, bring to the boil and simmer for 5 minutes.
Take off the heat, lay overlapping potato slices on top and then brush with melted butter or oil.
Replace the lid and bake for 1 hour.
Remove lid and place under a hot grill until potatoes start to go golden (about 8 minutes).
Serve on its own or with some crusty bread.