Preheat oven to 425°.
Thinly slice half of lemon; discard any seeds. Cut remaining lemon in half and then into 2 wedges.
Season chicken thighs with salt and pepper.
Coat a large skillet with 1 teaspoon oil and place on medium heat. Add chicken, skin side down. Cook allowing skin to render and brown, spoon off excess fat to retain a thin coating in pan, until chicken is cooked halfway through, about 10 minutes.
Scatter lemon slices over chicken and on bottom of skillet. Transfer skillet to oven, leaving chicken skin side down. Roast until chicken is cooked through, skin is crisp, and lemon slices on bottom of skillet are caramelized, 10-12 minutes.
Transfer chicken pieces, skin side up from the skillet to a platter.
Return skillet to medium heat. Add rosemary, onion and garlic; cook, stirring frequently about 2 minutes.
Add broth; cook until thickened, about 3 minutes. Squeeze 1 lemon wedge over and season sauce with salt, pepper, and juice from remaining lemon wedge, if desired.
Return chicken to skillet, skin side up, to rewarm.
Serve topped pan sauce.