2Heat olive oil and 2 tablespoons butter in a large oven-safe skillet over medium heat. Add onions, celery, and leek. Cook and stir until softened, about 10 minutes. Stir in lemon zest, thyme, garlic, and red pepper flakes. Cook until flavors combine, about 5 minutes. Mix in parsley. Remove from heat; transfer vegetable mixture to a bowl.
3Butterfly the chicken by removing the backbone, and pressing down on the breastbone to crack chicken open using a knife or kitchen shears. Open the 2 sides and spread them out like an open book. Rub the remaining 2 tablespoons butter all over the chicken. Season with salt and pepper.
