Step 1Trim the tough green tops of the leeks, leaving only the white stalk behind. Cut each stalk in quarters lengthwise, cutting down to but not through the root end. Rinse well under cold running water, separating the layers of the leeks to get rid of any dirt that might be hiding there. Thinly slice both leeks crosswise.
Step 2Melt the butter in a heavy 4- to 6- quart soup pot over medium-low heat. Add the onions and cook until just softened, about 3 minutes. Add the leeks, carrots, parsnips and rutabaga, cover tightly and cook gently until the vegetables are bright in color, beginning to soften and become aromatic, about 5 minutes. The vegetables do not need to be added all at once; you can chop them one at a time (they should be chopped to about the same size) and add them to the pot as you go along. Add the garlic and cook about 2 to 3 minutes, until fragrant.
Step 3Place the thyme sprigs in the center of the bay leaf and fold the bay leaf around them. Tie with string to hold together in a packet. Or you can fold the bay and thyme in a square of cheesecloth and tie it closed. Add the herb packet to the soup and cook for a minute or two.
Step 4Add 8 cups of water, 2 teaspoons of salt and a generous grinding of black pepper. Raise the heat and bring to a simmer. Partially cover the pot, leaving the lid ajar, and reduce the heat to maintain a sprightly simmer. The liquid should be bubbling quickly, but not boiling
Step 5Cook until the vegetables have softened and their flavors have married, about 1 hour. You should not taste any individual vegetable, but a more complex combination of all of them.
Step 6Cut the cabbage in lengthwise quarters and cut out the solid core. Cut each quarter in half lengthwise and then slice it about one-fourth to one-half inch thick. When the soup vegetables are cooked, add the cabbage to the pot and gently stir it in. Continue to simmer until the cabbage is silky and sweet, about 30 minutes.
Step 7While the soup is cooking, in a separate medium saucepan, bring 6 cups of water and 1 tablespoon salt to a rolling boil. Add the lentils, reduce the heat to a simmer and, with the cover slightly ajar, cook until they are tender, about 45 minutes. Drain and set aside.
Step 8When you are almost ready to serve, remove the bay leaf bundle from the soup. Raise the heat under the soup to a faster simmer and add the lentils. Stir gently to avoid breaking up the root vegetables. Stir in the vinegar and let the soup cook another minute or two to lose the raw smell. Season with a generous grinding of black pepper. Taste and adjust seasoning, adding more salt, pepper or vinegar as needed, then ladle the soup into warm bowls. Serve immediately.