Place the potatoes in a saucepan, and add enough water to cover. Bring to a boil and cook until tender; about 20 minutes. Drain, leaving the potatoes in the saucepan.
Meanwhile, stir the stock cubes into the warm milk until dissolved. Stir the milk mixture and butter into the potatoes. Mash the potatoes until smooth and creamy.
Heat the cooking oil in a frypan over medium-high heat. Prick the sausages in a few places with a fork, and place them into the pan. Cook, turning often, until golden brown and juices run clear-12 to 15 minutes depending on thickness. Remove from the frypan and drain on paper towels.
Using the same frypan, add the onion rings. Cook and stir over medium heat until the onions are golden, but still crisp, about 5 minutes.
Mix the gravy mix with the water and heat till just boiling in a small saucepan.
To serve, scoop the potatoes onto a large serving plate. Arrange the sausages over the potatoes, top with the onions, drizzle with gravy and sprinkle with oregano.