4 to 8 cups radish tops (to taste)
1 tablespoon butter or olive oil
1 onion, thinly sliced
1 large russet potato (about 1 lb), scrubbed, quartered, and thinly sliced
4 cups water or chicken stock
Juice of 1 lemon
Sea salt and freshly ground pepper
Few tablespoons thinly julienned radishes
1. Sort through the radish tops, tearing off and discarding thick stem ends or leaves that are less vibrant.
2. Melt the butter in a wide soup pot over medium heat. Add the onion slices, lay the potatoes over them, and cook for several minutes without disturbing them while the pan warms up. Then give them a stir, cover the pan, and cook over low heat for 10 to 15 minutes, stirring occasionally to keep from sticking. Add 2 tablespoons salt and the water or stock, and bring to a boil, scraping the pan bottom to dislodge any of the glaze. Lower the heat to simmer, and cook until the potatoes are tender and falling apart, about 15 minutes. Add the radish greens to the pot and cook long enough for them to wilt and go from bright to darker green, which will take just a few minutes.
3. Let the soup cool slightly, then puree it, leaving a bit of it rough if you like some texture, then return the soup to the pot. To finish, add the lemon juice and season with salt and pepper to taste.
4. Ladle into shallow bowls and stir a spoonful of yogurt into each. Scatter the julienned radishes over the top and serve.
Recipe from Deborah Madison's Vegetable Literacy.