4-8c radish tops (4-8 oz)
1 T butter or olive oil
1 onion, thinly sliced
1 lg russet potato (about 1 lb), scrubbed, quartered, and thinly sliced
juice of 1 lemon
sea salt and freshly ground pepper
few tablespoons thinly julienned radishes
Sort through the radish tops, tearing off and discarding the thick stems that don't have much leafy material and discarding any leaves that are less than vibrant.
Melt the butter in a wide soup pot over medium heat. Add the onion slices, lay the potato slices over them, and cook for several minutes without disturbing them while the pan warms up. Then give the onion and potato slices a stir, cover the pan, and cook over low heat for 10-15 min, giving the vegetables an occasional shove around the pan. The pan should take on a nice brown glaze from the onions. Add 2 t salt and the water and bring to a boil, scraping the pan bottom to dislodge any of the glaze. Lower the heat toa simmer, cover, and cook until the potatoes are tender and falling apart, about 15 min. 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.
Let the soup cool slightly, then puree it, greens and all, leaving it a bit rough if you like some texture or making it smooth if you prefer, then returne the soup to the pot. To finish, add the lemon juice and season with salt (potatoes can take a lot of salt) and pepper.
Ladle the soup into shallow bowls and stir a spoonful of yogurt into each bowl. Scatter the julienned radishes over the top and serve.
(From Deborah Madison's book Vegetable Literacy)