Here's a healthy, delicious soup that takes the chill out of a cold day. Serve with crusty bread for a satisfying lunch or light supper.
Separate the chard leaves from the stems. Thinly slice the stems (stalks) and cut the leaves crosswise into 1/2-inch-wide ribbons.
Heat the oil in a large saucepan or Dutch oven over medium-high heat. Add the carrot, celery, fennel, leeks, and chard stems; cook, stirring frequently, until the vegetables soften, 5 to 10 minutes.
Add the stock and bring the soup to a simmer. Add the potatoes and simmer the soup, covered, over medium heat until the potatoes are tender, 15 to 20 minutes. Uncover; with a fork, mash some of the potatoes against the side of the pan to thicken the soup.
Add the chard leaves and tomato to the soup, and cook until the chard is tender, about 5 minutes.
Stir in the fennel, nutmeg, and lemon zest. Season to taste with salt and pepper, and serve hot.