For each cup of mashed potatos, add about 1/4 cup of sour cream.
For a more complex dish, try this: (Sorry, these are U.S. measures and temperatures)
5 pounds small red-skinned potatoes
1 to 1 1/2 cups milk
2 tablespoons unsalted butter
4 tablespoons extra-virgin olive oil
2 large onions, minced
1/2 cup minced Italian parsley
2 large cloves garlic, minced
1/2 cup minced fresh basil leaves
1/2 cup water
Salt and freshly ground black pepper
Olive oil
1 cup (4 ounces) freshly grated Italian Parmigiano-Reggiano cheese
Cook and mash the potatoes as usual. (Potatoes should be smooth). While they are cooking, saute' the onions and garlic in the olive oil, just to transluscence. Add all but a smidgen of the parsley and all of the basil. Season lightly with the salt and pepper. Now, add the water and scrape the good brown bits from the bottom. Place 1 cup of the milk, all the butter and 1 tabelspoon of the olive oil in a bowl and add the recently saute'd flavorings. Blend in the mashed potatoes thoroughly. If the potatoes are a little dry use more milk, but sparingly. Mixture should be creamy and smooth but should form peaks and not be runny at all.
Now comes the good part... Using the olive oil, coat the inside of a 2 1/2 quart baking dish and use 1/2 of the Parmesan-Reggiano to cover the bottom of the dish. Turn the potato mixture into it, leveling as necessary, and cover with the remaining cheese. Cover with aluminum (aluminium, if you insist) and bake in a 350 degree F oven for about half an hour. Center of mixture should be heated thoroughly. Uncover and, just before serving, sprinkle with the remaining Parsley.
We often uncover the mixture and use the broiler to brown the top of the cheeses... being careful not to scorch it...