Cook potato cubes in boiling water for 15 to 20 minutes until tender. Drain and return to pot and cover.
Heat oil in a 2 quart saucepan and lightly brown the leeks and garlic. Add fresh rosemary and continue to cook 2 minutes. Add wine or chicken broth. Using and electric mixer, mix in potatoes. Slowly add sour cream and milk. Season with salt and pepper to taste.
Note: potatoes can be mashed manually, then stirred in by hand, as well as the sour cream and milk.