Season the chicken thighs on both sides with 1 teaspoon of salt and all of the pepper. Dredge the chicken thighs in flour to coat lightly. Once Skillet is heated, add the olive oil. Place chicken in hot Skillet. Brown on both sides, about 5 to 7 minutes, per side. It is important not to move the chicken when it is first placed in pan; chicken will release from Skillet once it is browned. Reserve.
Reduce heat to 350°F. Sauté onions and garlic for 2 to 3 minutes until softened. Stir in the rosemary sprigs and remaining ¼ teaspoon of salt. Add the lemon juice to the Skillet, scraping up any brown bits that remain on the cooking surface with a wooden spoon. Let juice come to a boil and reduce by half. Add the chicken broth and lemon zest and let come to a boil.
Nestle the browned chicken thighs in the onion mixture. Place lemon slices on top of the chicken. Reduce heat again to 300°F. Cover and cook until thighs are cooked through, about 45 to 50 minutes.
Garnish with rosemary; serve in shallow bowls with rice or mashed potatoes.