Rinse chicken inside and out under cold running water then blot dry inside and out with paper towels.
Place the chicken on its breast. Using poultry shears and starting on the neck end, cut out the backbone, making one lengthwise cut on either side. Remove and discard backbone. Remove breastbone. Cut off the wing tips and trim any loose skin. Pull down on legs to lay them flat. Season chicken generously on both sides with salt and pepper. Paint both sides with mustard.
Place bread crumbs, tarragon, garlic and hot red pepper flakes in a large shallow bowl or baking dish and stir to mix the chicken skin side down in the crumbs, then turn it over and do the same on the bone side.
When ready to cook, brush and oil the grill grate. Place the chicken, skin side up in the center of the hot grate, over the drip pan and away from the heat. Drizzle butter over the top of chicken. Cover the grill and cook the chicken until the crust is a golden brown and the meat is cooked through, about 40 minutes. If the crust starts to brown too much, loosely tent the bird with the aluminum bird with aluminum foil.