Prepare chicken by removing any packed organs, rinsing thoroughly, and patting dry. Generously sprinkle kosher salt on bird, rubbing into cavity and skin. Refrigerate for 1 hour.

After bird has rested, rinse away the salt mixture and pat dry. Position the chicken with the cavity facing you and use a pair of kitchen shears to cut horizontally through the rib bones below the breasts, from the cavity toward the front on both sides. Do not cut all the way through; the breasts should still be attached by cartilage and skin at the front. Swing the entire breast section up from the rear to the front, like opening the hood of a car, exposing the ribcage and backbone of the bird (watch the demo below). Sprinkle generously with kosher salt and pepper, or use spice mix. Flip the bird over and flatten with the heel of your hand at the top of the breasts. The meat and skin will now all be on the top, and the bird will be flattened. Tuck wings under body.

Combine the herbes de Provence and the butter to make an herb-butter mixture. Using your fingers to work the skin loose from the meat, spread half the butter under the skin of the breast. Work the rest into the skin of the bird, then apply spice mix generously over the entire skin.

Preheat oven to 400°, or prepare a Weber grill for indirect heat (bank coals to one side of grill). If roasting, place flattened chicken on a rack in a rimmed baking sheet lined with heavy-duty foil curled at the edges to contain juices. If grilling, form foil into a container shape or use an aluminum tray placed under the grill rack beside banked coals to catch the drippings. Place chicken on rack above foil.

Cook until temperature at the thigh reads 170° and juices are clear, 45 minutes to 1 hour. Let bird rest for 10 minutes. Serve on large platter, pouring accumulated juices over bird.