1. Preheat oven to 325ºF. In a Dutch oven or other large, ovenproof pot with a lid, cook sausage over medium heat, stirring to break it up, until crumbly and browned, 6 to 8 minutes. Spoon sausage into a bowl lined with paper towels and pour fat out of pot.
2. Return pot to heat and add oil. Add onion and mushrooms, season with salt and pepper and cook, stirring, until onion is soft and mushroom juices have evaporated, 8 to 10 minutes. Pour in wine and cook until reduced by half, stirring to release any browned bits on bottom of pan, about 2 minutes. Stir in rice and cook for 3 minutes. Pour in 3 cups of broth and stir to combine.
3. Bake for 20 minutes. Stir in Parmesan, reserved sausage meat and remaining 1 1/2 cups broth. Cover, return to oven and bake until rice is tender but not mushy, 30 to 35 minutes more. Sprinkle with parsley and serve.