In a large saucepan or soup pot, melt butter over medium-high heat. Add leeks and garlic; saute about 5 minutes or until tender. Add chicken broth, frozen potatoes, and the tomatoes. Bring to a boil. Reduce heat and simmer for 15 minutes. Working in batches, puree soup in a blender, filling the blender only half full for each batch. (Soup can be stored in refrigerator for up to two days at this point.)
Return soup to pan. Stir in cream and heat through over low heat. Season to taste with salt and pepper. Garnish servings with sour cream and chopped scallions.