2. Heat the oven to 425° F and put a 12-inch cast-iron skillet or heavy equivalent sauté pan inside it.
3. In a food processor, combine the picked herbs, lemon zest, garlic, spring onion and pulse until the mixture is very fine. Add olive oil, pinch of salt, and pepper and continue to pulse until it becomes a paste. If you don't have a food processor, just chop up all the ingredients as finely as possible. Set aside.
4. To spatchcock the chicken, put it breast-side down on a secure cutting board with the neck end closest to you. Using kitchen shears, trim any excess skin and fat from the opening of the neck cavity.