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Preheat the oven to 400°F. Generously season the chicken with salt and pepper. In a cast-iron pan or heavy ovenproof skillet over medium-high heat, warm the oil. Place both chicken legs in the pan, skin side down, and cook, adjusting the heat to avoid burning, until the skin is deeply golden, 10 to 12 minutes. Transfer the legs, skin side up, to a plate.
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Reduce the heat to low, add the leek and garlic to the same pan, and cook, stirring occasionally, until softened, about 3 minutes. Add the wine and chicken stock. Nestle the chicken legs into the pan, skin side up.
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Transfer the pan to the oven and bake until the chicken is fork-tender and the meat starts to come away from the bone at the drumstick, 45 to 50 minutes.
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Carefully return the hot pan to the stove (the handle will be hot) and gently transfer the chicken, skin side up, to a plate. Turn the heat to medium and stir in the Swiss chard, peas, and asparagus. If the pan seems dry, add a few tablespoons of water or chicken stock. Simmer the vegetables until the asparagus is firm but cooked, about 3 minutes, then stir in the crème fraîche and lemon zest. Return the chicken, skin side up, to the pan. Sprinkle with tarragon leaves and serve.