Cut zucchini in half lengthwise; cut each half into 1/4-inch slices.
Microwave broth in a 2-qt. microwave-safe bowl at HIGH 6 to 7 minutes or until very hot.
Melt butter in a large Dutch oven over medium heat; add onion, and sauté 4 to 5 minutes or until tender. Stir in rice, and cook, stirring constantly, 2 minutes. Stir in hot broth, and bring to a boil over medium-high heat. Cover, reduce heat to low, and simmer 10 minutes. Stir in zucchini and carrots. Cover and cook 10 to 12 more minutes or until rice is tender. Stir in cheese and next 3 ingredients, stirring until cheese is melted. Remove from heat; cover and let stand 10 to 12 minutes or until liquid is absorbed. Garnish, if desired.