Melt 3 tbsp butter in lg. Saucepan. Add onion; cook on med-high heat until onion is translucent. Add rice; cook until rice is lightly browned, stirring frequently. Add carrot and broth. Bring to a simmer; cover and reduce heat to low. Cook, covered, 16 to 18 mins. Meanwhile, melt 2 tbsp butter in a skillet. Add mushrooms; cook on med-high heat, stirring frequently, until soft and lightly browned. Remove from heat. Uncover rice and fluff with fork; stir in mushrooms; serve.