1. Cook noodles, following the label directions; drain. Combine with onions and 1 can of the mushroom soup in a greased shallow 12-cup baking dish; sprinkle with peas.
2. While noodles cook, pull skin from chicken breasts; cut each in half. Arrange in a single layer over peas; spread with remaining can of soup; cover.
3. Bake in moderate oven (350°) 11/2 hours, or until chicken is tender and sauce is bubbly. Garnish with chopped pistachio nuts, if you wish.