Step 1Bring a small pot of water to a boil. With a vegetable peeler, peel the zest in strips from the orange, avoiding the white pith. Finely chop the zest, place in a small strainer and dip into the boiling water to blanch for 1 minute. Rinse the zest under cold water and drain it on a paper towel.
Step 2Spread the zest on a plate and allow it to dry at room temperature for 24 hours or in the microwave for 3 to 4 minutes. Set aside. Squeeze one-half cup juice from the orange. Set aside.
Step 3Cut off the carrot greens and reserve. Peel and trim the carrots. In a saute pan large enough to hold the carrots in a single layer, melt 2 tablespoons of butter over medium-high heat. Add the onion and saute for 30 seconds. Add the orange juice, sugar, a pinch of salt, the coriander and carrots. Bring to a simmer, then cover, reduce the heat and simmer for 5 minutes.
Step 4Add the chicken stock and simmer until the carrots are tender when pierced with the tip of a knife, about 15 minutes.
Step 5Meanwhile, bring a medium pot of water to a boil and fill a medium bowl with ice water. Pick the leaves from the carrot greens and discard the stems. Break the leaves into 1- to 2-inch pieces. There should be about 1 cup of loosely packed greens. Blanch the greens in the boiling water for about 15 seconds. Drain in a strainer and immediately plunge the strainer into the ice water to stop the cooking. When the greens are cold, lift the strainer, drain well, and dry the greens on paper towels.
Step 6When the carrots are tender, transfer them to a plate and cover loosely to keep warm. If there is more than one-half cup of liquid remaining in the pan, return the pan to the burner and simmer to reduce it to one-half cup. Combine the cooking liquid and the remaining 2 tablespoons butter in a blender, add the carrot greens, and blend until smooth. Season to taste with salt and pepper. (There will be about one-half-cup sauce.)
Step 7Gently toss the mache with the oil, vinegar, sugar and salt and pepper to taste. Serve the carrots family-style on a platter, arranging them in a bunch with the mache at the top and the sauce poured along one side. Or serve two carrots per plate with a small mound of mache and the sauce spooned onto the side. Sprinkle the carrots with the orange zest.