Melt butter in a soup pot over medium heat. Add turnips, potatoes, onion or leeks, garlic, parsley, and thyme. Cook for about 5 minutes, add 1 teaspoon salt and water or broth, and bring to a boil. Reduce heat and simmer, partially covered, until vegetables are tender, about 25 minutes. Keep the soup chunky if you like, purée a portion of it (such as 1 cup or half) to give it a creamy background, or purée the whole thing.