Cook rice according to package directions, discarding spice packet.
Spray medium skillet with cooking spray; heat over medium heat until hot. Add onion, green and red pepper, mushrooms, and Italian seasoning; cook, covered, over medium heat until mushrooms are wilted, about 5 minutes. Cook, uncovered, until vegetables are tender and excess moisture is gone, 5 to 8 minutes.
Combine rice, mushroom mixture, chicken, beans, corn, peas, sour cream and 1 cup cheese; season to taste with salt and pepper. Spoon into 2-quart casserole; sprinkle with remaining ½ cup cheese. Bake, uncovered, at 350° F. until hot through, about 30 minutes.