Season chicken with salt and pepper. Heat oil and lemon slices in a 12″ cast-iron or heavy nonstick skillet over high heat until hot but not smoking. Nestle chicken in skillet, skin side down, and cook 2 minutes. Add thyme sprigs. Reduce heat to medium-high; continue cooking skin side down, occasionally rearranging chicken thighs and rotating pan to evenly distribute heat, until fat renders and skin is golden brown, about 12 minutes.