Preheat oven to 160˚C fan-forced. Heat 1 tbs oil in a large ovenproof pan over medium-high heat. Brown chicken in 2 batches. Transfer to a plate. Set aside.
Heat remaining 1 tbs oil in the pan over medium heat. Add onion and garlic. Cook, stirring often, for 3-4 minutes until onion softens. Toss through mushrooms. Cook for 1 minute.
Stir in rice. Cook, stirring, for 1-2 minutes or until rice is glossy. Add stock and return chicken to pan. Stir to combine and bring to the boil. Cover and bake for 25-30 minutes or until rice is just tender and liquid has almost been absorbed. Toss through spinach. Sprinkle with parmesan. Serve with lemon wedges.