From the New York Times, with a few adjustments
1 tablespoon extra virgin olive oil
1 medium onion, chopped
1/2 pound leeks (1 large or 2 small), white and light green parts only, cleaned well and sliced
1/2 pound carrots (2 large), peeled and sliced
1/2 pound kohlrabi, trimmed, peeled and diced (1 large bulb)
1/2 pound turnips, peeled and diced (2 medium)
6 ounces potatoes (2 medium), peeled and diced
1 1/2 quarts vegetable stock
2 teaspoons ground ginger
1 bay leaf
1 teaspoon each dried thyme and parsley
Salt and freshly ground pepper to taste
Heat the olive oil over medium heat in a large, heavy soup pot or Dutch oven. Add the onion. Cook, stirring, until it begins to soften, about three minutes. Add the leeks and a generous pinch of salt. Cook, stirring often, until tender but not browned, about five more minutes. Add the carrots, kohlrabi, turnips, potatoes and water or stock. Bring to a boil.
Add seasonings and salt to taste (about 2 teaspoons), reduce the heat, cover and simmer one hour.
Blend the soup until smooth with an immersion blender or in batches in a regular blender. (Do not put the top on tight; cover the top with a towel to prevent hot splashes.) Add lots of freshly ground pepper, taste and adjust salt, and serve.