Heat the oven to 375ºF.

• Trim the celery root. Using a vegetable peeler or knife, remove the peel and cut into ½-inch-thick pieces.
• Scrub the parsnips, trim the tops, and cut into ¼-inch-thick slices.
• Finely chop the dill leaves. Discard the stems.

In a sauce pot over medium heat, warm 2 tablespoons oil until hot but not smoking. Add the celery root and parsnips, season with salt and pepper, and cook, stirring occasionally, until they start to soften, 3 to 4 minutes.
Add 1 cup water and bring to a boil. Reduce to a simmer, cover, and cook until most of the liquid has evaporated, 10 to 12 minutes.
Remove from the heat, and with a vegetable masher, blend until mostly smooth. Add the dill, and season with salt and pepper. While the celery root and parsnips cook, prepare the remaining vegetables.

• Trim the green tops from the leeks. Cut the white portion into ¼-inch-thick half-moon slices. Rinse.
• Wash the radishes and cut into quarters.
• Finely chop the parsley leaves.

• Season the chicken with salt and pepper.

In a frying pan over medium-high heat, warm 1 to 2 tablespoons oil until hot but not smoking. Add the chicken, skin-side down, and cook until the skin is crisp, 3 to 4 minutes. Turn the chicken and cook for another minute. Transfer to a sheet pan and roast in the oven until the chicken is cooked through, 12 to 15 minutes.

In the same pan, pour off 1 teaspoon of the fat, add the leeks, season with salt and pepper, and cook over medium heat, stirring occasionally, until softened, 2 to 3 minutes.
Add the radishes and cook for 1 minute. Add the chicken stock and bring to a boil.
Reduce to a simmer and cook until the radishes are tender, 8 to 10 minutes.
Remove from the heat and add the butter, if using. Stir in the parsley and season with salt.

Transfer the chicken and celery root puree to individual plates. Top with the radishes and serve.