Preheat the oven to 400. Cover the potatoes with cold water in a saucepan and salt the water. Bring to a boil over medium-high heat and cook until tender, about 8 minutes; drain and set aside.

Pile the rosemary leaves, garlic, 2 teaspoons salt and the red pepper flakes on a cutting board, then mince and mash into a paste using a large knife. Transfer the paste to a bowl. Stir in the juice of 1 lemon and the olive oil. Add the chicken and turn to coat.

Heat a large cast-iron skillet over medium-high heat. Add the chicken, cover and cook until it browns, about 5 minutes. Turn the chicken and cook on the other side for a couple more minutes.

Take a large baking dish and brush the remaining tbsp of olive oil over the bottom of the baking dish. Add the chicken breasts in the middle and the potatoes around the chicken. Throw in the mushrooms.

Add the rosemary sprigs and the squeezed lemon halves to the baking dish and drizzle the juice of the remaining lemon, plus any additional garlic lemon juice mixture left; transfer to the oven and roast, uncovered, until the chicken is cooked through and the skin is crisp. I kept mine for about an hour.