This cold snap makes me want soup and potato leek soup is among my favorites. You can build upon this basic soup, adding any vegetable you like. I like turnips, diced tomatoes, carrots, etc.
2 large leeks, trimmed, cleaned, and cut up
1 tablespoon butter or olive oil
3 medium potatoes, any type, peeled and cut into small cubes
Salt and freshly ground pepper to taste
4 cups stock; I like full flavored chicken, but you can use vegetable or beef