Preheat the oven to 450 degrees . In a large ovenproof skillet, heat 1 tbsp. EVOO over medium-high. Season the chicken with salt and pepper and cook, skin side down, until golden-brown, about 7 minutes. Turn chicken, scatter with the lemon slices, rosemary sprigs and garlic, and roast until cooked through, about 25 minutes.
Meanwhile, in a large skillet, heat the remaining 1 tbsp. EVOO over medium-high. Cook the cauliflower, stirring occasionally, until browned in spots, about 5 minutes. Add 3/4 cup water, partially cover and cook until just tender, 10 minutes. Season.
Transfer the cauliflower to a platter and add the chicken and lemon slices. Discard the rosemary. Mash the garlic into the pan juices. Top the chicken with the pan sauce and sprinkle with parsley.