Preheat the oven to 425 degrees F. Set an oven rack to one notch below the center position. Coat a baking sheet with non-stick cooking spray or line it with parchment paper. Set aside.
To a medium mixing bowl, add the potatoes, carrots, and onions. Drizzle with olive oil and add the dried parsley and salt. Gently toss to coat the veggies and then set aside.
In a small bowl, mix together the softened butter, chopped fresh parsley, garlic, and pepper until evenly combined.
Place the flattened chicken on the prepared baking sheet with the breasts facing up. Insert your hands between the skin of the chicken and the meat to release the skin. Then rub half of the compound butter under the skin and the other half on top of the skin.
Arrange the vegetables on the baking sheet around the chicken. Sprinkle the lemon zest and juice all over the baking sheet.
Roast for 40 minutes or until an instant-read thermometer inserted in the thickest part of the breast reads 165 degrees F internal temperature.
Remove from the oven and allow to rest for 15 minutes. Then cut the chicken into quarters (breast + wing and leg + thigh). Serve each chicken quarter with some of the roasted veggies. Sprinkle with chopped parsley, if desired.
Notes
You can use a 3 to 4-pound chicken for this recipe. Just be sure to double-check the internal temperature of the chicken at the end of the cooking time.
You can find a pre-spatchcocked chicken at some grocery stores. If needed, you can prep the chicken yourself using this easy to follow prep video.
If you don't have salted butter on hand, add 1/8 teaspoon of salt to the compound butter mixture.