How to Make It
Preheat the oven to 375°. In a small skillet, cook the butter over moderately low heat until golden brown, about 7 minutes. Pour the brown butter into a bowl.
Season the chickens with salt and pepper. In a large, flameproof roasting pan, heat the grapeseed oil. Add the chickens and cook over moderately high heat, turning occasionally, until lightly browned all over, about 7 minutes. Turn the chickens breast side up and scatter the garlic and thyme all around. Drizzle 2 tablespoons of the brown butter over the breasts. Roast the chickens for about 1 hour and 10 minutes, or until an instant-read thermometer inserted into an inner thigh registers 165°. Let the chickens rest for 10 minutes. Pour the pan drippings into a bowl and skim off the fat. Discard the garlic and thyme.
Meanwhile, in a small saucepan, combine the orange juice and vinegar and boil over moderately high heat until reduced to 2 tablespoons, about 7 minutes.
In a large, deep skillet, heat the remaining 6 tablespoons of brown butter. Add the carrots, parsnips and radishes and cook over moderately high heat, stirring until tender, about 12 minutes. Add the orange glaze and the chicken drippings and cook, stirring, until the vegetables are glazed, 1 minute; season with salt and pepper.
Carve the chickens. Transfer to plates and serve with the vegetables.