In 4-cup (1 L) measure, microwave stock at High for 3 to 4 minutes or until simmering.
In 12-cup (3 L) deep casserole, combine butter and onion.
Microwave at High for 2 minutes, or until onion has softened, stirring once.
Add rice; stir to coat rice with butter.
Pour in hot stock; cover with lid or vented plastic wrap and microwave at High for 4 to 5 minutes or until boiling.
Microwave at Medium (50%) for 7 to 9 minutes, or until rice is just tender and mixture is still creamy, rotating once if necessary.
Rinse peas with warm water to thaw and stir into rice mixture.
Cover and let stand 5 minutes.
Stir in cheese.
Taste and adjust seasoning with salt and pepper.