Heat the olive oil in a large skillet over medium-high heat. Add the onion and reduce the heat to medium-low; cook until soft but not brown, about 5 minutes. Stir in the rice and cook and stir until the outside of the grains are translucent, 3 to 5 minutes.
Coat the inside of a 4-quart slow cooker with cooking spray. Add the rice mixture, broth, and salt; stir to combine. Cover; cook on high-heat setting for 1 hour, 15 minutes.
Just before the rice is done, using the same skillet, cook the sausage over medium heat until browned, 2 to 3 minutes.
Stir the sausage, peas, Parmesan, butter, and pepper into the rice mixture. Turn off cooker; let mixture stand, covered, for 10 minutes. Stir before serving. Top with additional Parmesan, if desired.