How to Make It
In a 10-inch skillet, combine all of the ingredients and bring just to a simmer. Cook over moderately low heat until the potatoes are just tender, about 15 minutes. Let cool. Drain the potatoes, discarding the garlic and peppercorns; reserve the oil.
In a large saucepan, heat 1/3 cup of the reserved potato oil. Add the garlic and half the shallots and cook over moderate heat, turning occasionally, until softened, 5 minutes. Add the thyme sprigs and crushed pepper and cook until fragrant, about 30 seconds. Add the clams, wine and a pinch each of salt and pepper. Cover and cook over high heat, shaking the pan, until the clams open, 5 minutes. Remove the clams and discard the shells. Finely chop 1/3 cup of the clams. Strain the broth; reserve for another use.
In a medium bowl, whisk the vinegar with the chopped clams, walnuts, chervil and thyme and the remaining shallot; let stand for 15 minutes. Season the dressing with salt.
In a large skillet, heat 2 tablespoons of the reserved potato oil until shimmering. Add the potatoes cut side down and cook over moderately high heat, turning once, until lightly browned, 3 to 5 minutes. Transfer to a platter. Add the radishes to the skillet cut side down and cook until browned on the bottom, about 3 minutes. Transfer to the platter, fold in the radish greens and season with salt.
Add the dressing and the whole clams to the skillet and warm over moderately low heat, about 3 minutes. Spoon over the potatoes and radishes, top with the dill and serve.