1. Cook rice according to package directions.
2. Sprinkle chicken evenly with salt and pepper. Sauté, in 2 batches, in 2 Tbsp. hot oil in a large skillet over medium-high heat 5 to 7 minutes or until lightly browned. Remove from skillet.
3. Add remaining 1 Tbsp. oil to skillet, and sauté onion 2 to 3 minutes or until tender. Stir in mushrooms, and cook 2 to 3 minutes or until liquid evaporates. Remove skillet from heat.
4. Stir together soup, sour cream, and Italian seasoning in a medium bowl until well blended. Stir soup mixture into onion mixture in skillet; stir in rice. Gently stir in tomatoes and chicken just until combined. Spoon mixture into a lightly greased 13- x 9-inch baking dish.
5. Bake at 350° for 25 to 30 minutes.
To make ahead: Line a 13- x 9-inch baking dish with heavy-duty aluminum foil, allowing several inches of foil to extend over sides. Prepare recipe as directed through Step 4; spoon mixture into dish. Freeze 2 hours or until firm. Lift casserole from dish using foil sides; fold foil over casserole. Wrap in additional foil, or place in a large zip-top plastic freezer bag. Freeze up to 3 months. Remove casserole from foil or freezer bag. Place in a lightly greased 13- x 9-inch baking dish. Thaw in refrigerator 24 hours. Bake at 350° for 1 hour to 1 hour and 10 minutes or until bubbly.