How to Make It
Preheat oven to 400°F. Place the chicken breast-side down on a cutting board. Using a pair of heavy-duty kitchen shears, cut down both sides of the backbone to remove it. Flip the chicken so that the breast side is up; press down to flatten. Rub with 1 tablespoon of the oil and sprinkle evenly with the coriander, pepper, and ½ teaspoon of the salt. Heat 1 tablespoon of the oil in a 12-inch cast-iron skillet over medium-high. Place the chicken breast-side down in the skillet and top with a smaller heavy skillet. Cook until the skin is golden brown, about 5 minutes. Remove the smaller skillet. Transfer the chicken to a plate.
Toss together the onion, sweet potato, mushrooms, garlic, and the remaining oil and salt. Arrange the vegetables in a single layer in the skillet. Place the chicken breast-side up on the vegetables. Bake in oven until the chicken is cooked through and the vegetables are tender and lightly browned, about 50 minutes. Serve sprinkled with the oregano.