Make the compound butter: Stir all ingredients together very well. Set aside (don't refrigerate if you're using it right away).
Spatchcock the chicken: Place the chicken breast-side down in a large roasting tin in front of you, with the legs pointing towards you. Remove the backbone by cutting left and right of it through the ribs with shears. Turn the chicken around and press down on the breastbone until you hear it crack and the chicken lays flat (see post for images).
Prepare the chicken: Preheat the oven to 410°F (210°C). Place the thyme and rosemary sprigs in the roasting tin around the chicken, along with the cut garlic and lemon. Gently separate the skin over the chicken breast from the meat without ripping it. Fill the pockets with the prepared compound butter. Drizzle the chicken with olive oil and rub it with salt and pepper.
Roast the chicken: Place the chicken in the preheated oven and roast it for 15 minutes. Reduce the heat to 360°F (180°C) and roast until internal temperature reaches 165°F (75°C). This will take about 30 minutes. Start checking after 20 and keep roasting until the thickest part of your chicken reaches the safe temperature.
Make ahead: You can spatchcock a fresh chicken 2 days in advance, if you work cleanly. Simply keep it in a roasting tin on the bottom shelf of your fridge, well wrapped, so it doesn't drip or leak on your other food. You can also ask your butcher to do it for you and then lightly vacuum pack it. If you defrost a chicken in the microwave, you need to use it immediately.The compound butter can be made three days ahead and kept in an airtight container in the fridge.
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