To butterfly chicken, using kitchen shears or a long heavy knife, cut along both sides of backbone; remove and discard backbone. Rinse the chicken body cavity; pat dry with paper towels. Turn chicken, skin side up, and press down between the breast halves to break the beast bone. Place chicken in a 13x9x2-inch baking pan.
Chop 1/4 cup of the parsley, 1 tablespoon of the thyme, and 1 tablespoon of the rosemary. Gather the remaining parsley, thyme, and rosemary sprigs into a bundle; at the base of the bundle tie the sprigs together with 100 percent cotton kitchen string to make a brush. Set aside.
For marinade, in a medium bowl combine the chopped parsley, thyme, rosemary, 1/4 cup of the olive oil, 2 tablespoons of the lemon juice, the lemon slices, garlic, 1/2 teaspoon of the salt, 1/2 teaspoon of the black pepper, and 1/2 teaspoon of the crushed red pepper. Spread the marinade over chicken in baking pan. Use your clean hands to spread and massage the marinade all over the chicken until completely coated. Cover and marinate in the refrigerator for at least 2 hours or up to 24 hours.
Meanwhile, for brush-on mixture, in a small bowl combine the remaining 1/4 cup olive oil, the remaining 4 tablespoons lemon juice, the remaining 1/4 teaspoon salt, the remaining 1/4 teaspoon black pepper, and the remaining 1/4 teaspoon crushed red pepper. Cover and chill until ready to use.
Drain chicken, discarding marinade and lemon slices. For a charcoal grill, arrange medium-hot coals around a drip pan. Test for medium heat above the pan. Place chicken, skin side down, on grill rack over drip pan. Cover and grill for 1 to 1-1/4 hours or until chicken is no longer pink and drumsticks move easily in their sockets (180 degrees F in inside thigh muscle), turning and rotating chicken halfway through grilling time. Use the herb sprig brush to brush the brush-on mixture over chicken during the last 10 minutes of grilling time. (For gas grill, preheat grill. Reduce heat to medium. Adjust for indirect cooking. Place chicken on grill rack over burner that is off. Grill as above.) Discard herb sprig brush and any remaining brush-on mixture.