Carefully pour off all but 2 Tbs. fat from the pan. Return the pan to the heat, add the onion and sauté until the onion is soft and translucent, 4 to 6 minutes. Add the garlic and a pinch of pepper; continue to cook, stirring, for 30 seconds. Add the orzo and sun-dried tomatoes and sauté for another 30 seconds. Add the wine and deglaze, stirring to scrape up any browned bits from the pan bottom, and continue cooking until the liquid is reduced by half. Add the chicken stock and lemon slices, then return the chicken to the pan, skin side up. Transfer to the oven and cook, uncovered, until the chicken is cooked through and the orzo has absorbed the liquid, 35 to 40 minutes. Remove the pan from the oven and let sit for 5 minutes before serving.
