Creamy Potato Soup
["2 Tbsp. butter or margarine", "2 Tbsp. finely chopped onion", "1 1/2 tsp. salt", "1/4 tsp. celery salt", "1/8 tsp. pepper", "3 1/2 c. milk", "1 1/3 c. instant mashed potatoes (dry)"]
["Heat first 6 ingredients until milk is scalding.", "Stir in potatoes.", "Cook until smooth, stirring constantly."]
["butter", "onion", "salt", "celery salt", "pepper", "milk", "instant mashed potatoes"]