Preheat the oven to 425 degrees. In a small mixing bowl or dressing shaker, add the olive oil, lemon juice, chopped rosemary, garlic, and red pepper flakes. In a medium bowl, pour in half the marinade and add the chicken, season generously with salt and pepper and toss to coat the chicken in the seasonings.

Heat a large oven-safe (cast-iron or other) skillet over medium-high heat. Add the chicken and cook until browned, about 5 minutes. Flip the chicken and add the Brussels sprouts and potatoes to the skillet. Toss the potatoes and Brussels sprouts and place the chicken on top of the potatoes and Brussels sprouts. Pour over the remaining marinade over the skillet, along with the rosemary sprigs and used lemons. Transfer the skillet to the oven and let roast, uncovered, for 25 minutes or until chicken is cooked through (no longer pink on the inside.)

Remove from the oven, garnish with parsley, and serve.