Preheat the oven to 170 C / Gas 3. Place all the sausages in a roasting tin.
Roast for 45 to 60 minutes until coloured and most of the fat is in the tin.
As the sausages are roasting, finely chop the onion and gently fry it in the oil until translucent to golden brown. The longer they are gently fried, the better the caramelisation.
Once the onions are fried add the tomatoes and beef stock cubes. Add only one stock cube for a mild beef taste, the second one for a stronger flavour. Stir well and bring to the boil, then simmer on a low heat to thicken the sauce. Add the herbs, salt and pepper to taste.
Whilst the sauce is cooking, peel the potatoes, cut into quarters and boil. When soft, mash to a lump free mash.
Bring it all together: Remove sausages from roasting pan and place on paper towel to remove excess fat, evenly dish them out onto warm plates. Serve the mash onto plates and cover the whole lot with the reduced sauce.