Place potatoes in a medium pot and cover with water. Bring to a boil over high heat. Simmer over medium heat until potatoes are tender. Drain and set aside to cool.

Then peel and mash the potatoes with a potato masher.

Melt butter in a pot over medium heat. Add the mashed potatoes, milk, salt and pepper to taste. Stir well and cook for 5 minutes.

Meanwhile heat a non-stick skillet over medium-high heat. Add oil and sausage and cook until sausage is browned. Remove sausage from skillet.

Preheat the oven to 350°F. Grease an 8 x 11 inch baking dish with oil.

In a large bowl, combine the mashed potatoes, sausage, oregano and 1-cup of cheese. Reserve 1-cup of the cheese for topping.

Transfer to the baking dish and top with remaining cheese.

Place on the middle rack of the oven and bake for 25 minutes. Then put under broil until golden brown, about 2-3 minutes (keep an eye on casserole while broiling).