One of the things that excites me the most is to take a humble vegetable and turn it into something exquisite. Like... a simple radish. I love to serve this soup side by side with the roasted roots. Take a bite of the sweet, fennel-infused roots along with a spoonful of the spicy, silky green-top soup and you’ll agree: The humblest of vegetables can truly be transformed into a heavenly experience.
Note: The soup will keep for about 3 days in the refrigerator or 1 month in the freezer.
Preheat the oven to 425 degrees.
Place the radishes in a heavy-bottomed, nonstick roasting pan large enough to hold the slices in a single layer. Season with the fennel seeds, salt, and pepper. Drizzle the maple syrup and olive oil on top. Toss well, cover tightly with aluminum foil and bake until very tender, tossing once halfway through the cooking, about 45 minutes.
Remove the foil and toss the vegetables carefully so as not to break them. Return to the oven, uncovered, and bake until golden, about 12-15 more minutes. Remove from the oven and keep warm.
Heat a large, heavy-bottomed soup pot over medium-high heat. Add the olive oil, shallots, red pepper flakes, and garlic. Sauté until softened, stirring occasionally, about 2-3 minutes. Add the potatoes, stock, and water. Season with salt and pepper, to taste.
Bring to a boil over high heat. Add the radish tops, stir well, and continue to boil until wilted, about 1-2 minutes. Reduce heat to medium, cover the pot and simmer for 20 minutes until the potatoes are tender.
Add the crème fraîche and purée the soup with an immersion blender or food processor until silky smooth. Taste and adjust the seasoning if needed and keep warm. Place a small mound of the roots on a plate. Ladle the soup in a small bowl and place next to the roots. Drizzle the infused oil on the soup and serve immediately.