Preheat oven to 450. Cook potatoes in boiling, salted water until just tender, about 10 minutes.
Drain potatoes, shaking in a colander to rough the potatoes up a bit. Set aside.
Put salt, rosemary leaves, garlic, and red pepper flakes in a food processor or using a mortar and pestle, process into a paste. Add the juice of one lemon and the olive oil to combine.
Heat a 12" oven-safe non-stick skillet, or cast-iron skillet over medium-high heat. (If using cast-iron, add a tablespoon of olive oil).
Add the chicken to the skillet, and pour the rosemary/lemon mixture over it, turning to coat. Brown chicken, about 3 minutes on each side.
Add potatoes to the skillet, and toss them around until they start to turn golden. Squeeze the remaining lemon over the chicken and potatoes. Add the rosemary sprigs and the squeezed lemon halves to the skillet.
Transfer skillet to the oven and roast until chicken is cooked through and potatoes are nicely browned, about 15 minutes.
Notes
This recipe is easy to increase, or even double. Use more chicken pieces for a hungrier group, and add more potatoes. You can use chicken breasts instead of thighs, or chicken pieces. A side of steamed green beans or brocolli is a perfect accompaniment.