Cook noodles as directed on package, adding peas during last minute of cooking time; drain.
Meanwhile, heat oven to 350°F. In ungreased 2-quart casserole, mix soup, milk, cheese and onion.
Stir cooked noodles and the tuna into soup mixture. Cover; bake 30 minutes.
Uncover; stir mixture well. Sprinkle with crackers. Bake uncovered about 5 minutes longer or until crackers are light golden brown.