Preheat your oven to 425°F (220°C). Place the whole chicken breast-side down on a cutting board. Using a sharp knife, cut along the backbone and flip it over. Press down firmly on the breast to flatten the chicken for even cooking.
In a bowl, combine olive oil, yellow mustard, honey, crushed garlic, dried rosemary, crushed red pepper flakes, salt, and pepper. Squeeze the juice of the medium lemon into the mixture and add the fresh lemon zest. Whisk until well combined.
Use a basting brush to generously coat the flattened chicken with the marinade, including under the skin. If possible, marinate for at least 30 minutes or up to overnight in the refrigerator.
If using, slice the top off the head of garlic to expose cloves and place it in the roasting pan alongside the chicken to infuse flavor.
Place the marinated chicken breast-side up in the roasting pan. Roast for about 1 hour and 15 minutes, or until juices run clear and a meat thermometer inserted into the thickest thigh reads 165°F (75°C).
Remove chicken from oven and let rest for 10-15 minutes to allow juices to redistribute. Garnish with fresh parsley and lemon wedges before serving.