Position the oven rack 8 inches (middle rack for most ovens) from the flame/coil and turn broiler to high.
Make a gremolata: Coarsely grind the peppercorns in a mortar and pestle. Work in the garlic and 1/2 teaspoon of the salt. Follow with the lemon zest, parsley, and finally 2 tablespoons of the oil. Work into a rough paste.
Arrange vegetables in a heavy roasting pan, creating a bed for the bird.
Place chicken on a plastic cutting board, breast-side down. Using kitchen shears, cut ribs down one side of backbone and then the other and remove, saving the backbone for another application, like stock. Open chicken like a book and remove the keel bone separating the breast halves by slicing through the thin membrane covering it, placing two fingers underneath the bone, and levering it out. Turn chicken breast-side up and spread out like a butterfly by pressing down on the breast and pulling the legs towards you. Loosen the skin at the neck and the edges of the thighs. Evenly distribute the garlic mixture under the skin, saving 2 teaspoons for the jus. Drizzle the skin with oil and rub in, being sure to cover the bird evenly. Drizzle oil on bone side of chicken, as well.
Place the bird, skin-side down, on veggie bed and sprinkle with another 1/2 teaspoon of the salt. Move to oven and broil 25 minutes.
Carefully flip bird, season skin with remaining 1/2 teaspoon of salt, and return to oven until the deepest meat of the thigh and breast reach 160ºF, about another 15 to 20 minutes.
Remove the bird to a wire rack set over a paper towel-lined half-sheet pan to rest. Position roasting pan on cooktop so that it straddles 2 burners — both cranked to high. Add the wine and the broth. Push the vegetables around to loosen any stuck-on bits of goodness. Discard the veggies — they’ve given their all. Continue boiling the liquid for 2 minutes or until reduced to 1 cup.
Drain liquid into gravy separator. Carve the bird while the fat floats to the top. What’s left in the bottom is jus. Sauce the chicken with the jus and garnish with reserved gremolata.