Preheat oven to 375 degrees. Heat large nonstick skillet over medium heat until hot. Add ground beef and mushrooms; cook 8 to 10 minutes, breaking into 3/4-inch crumbles and stirring occasionally. Remove drippings. Stir in rice, contents of seasoning packet, and tomatoes. Spoon beef mixture into 2-quart baking dish. Stir in water amount indicated in rice package directions, plus 1/2 cup additional water. Cover with aluminum foil.