Heat oven to 350°. Cook noodles according to package directions for 4 minutes; drain. Meanwhile, in large skillet, cook meat and onion; drain. Stir in thyme, salt and pepper; place in large bowl. Add mushrooms, tomatoes, sour cream and noodles. Spoon mixture into greased 2-quart casserole; sprinkle with cheese. Bake, uncovered, 30 minutes or until hot and bubbly.