Bring broth to a boil in medium saucepan over high heat. Reduce heat to low to keep broth barely simmering.
Meanwhile, heat oil in 3-quart saucepan over medium-high heat until hot. Add mushrooms, onion and garlic; cook and stir 2 minutes. Add rice; cook and stir 1 minute.
Carefully stir in 1 cup hot broth and salt. Bring to a boil. Reduce heat to medium-low; cook and stir until almost all of liquid is absorbed. Continue adding broth, 1/2 cup at a time, cooking and stirring after each addition until liquid is absorbed. Rice should be tender and creamy but still slightly firm in center.
Add spinach and artichoke hearts; cook and stir 2 to 3 minutes or until spinach is wilted and artichokes are thoroughly heated. Stir in cheese. If desired, garnish with additional Parmesan cheese.