Preheat the oven to 180C/350F/Gas 4.
In a large saucepan of boiling, salted water, boil the potatoes for about 15-20 minutes, until tender. Drain.
Add the butter and a pinch of salt to the potatoes. Mash well, using a potato ricer, mouli or traditional masher, adding enough milk or cream to bring it to a creamy consistency.
Roll the sausagemeat into walnut-sized balls and place in an oven-proof pie dish. Transfer to the oven and bake for 20-25 minutes until sizzling, but not brown.
Put the onions in a frying pan with the olive oil and cook until softened. Add the tomatoes, herbs, and salt and pepper to taste. Bring to the boil, before simmering for 10 minutes.
Pour the tomato sauce over the cooked sausage balls and top with the mashed potato.
Sprinkle with grated cheese if liked. Return to the oven and cook for 30 minutes, until golden brown. Serve with peas and sweetcorn.