Found this handwritten on an index card in my mother-in-law’s recipe box dated 1968.

If you want to stay true to the old index-card spirit but still tweak things a bit, you can swap the cream of mushroom soup for cream of chicken or cream of celery for a slightly different flavor while keeping it to three ingredients. For a bit more tang, use baby Swiss or a mix of Swiss and provolone, as long as you still only use one kind of cheese. If your chicken breasts are very thick, you can cut them in half horizontally so they cook more evenly and stay tender.

For make-ahead prep, you can assemble the dish up to a day in advance, cover tightly, and refrigerate; add 5 to 10 minutes to the covered baking time if starting from cold. Always handle raw chicken carefully: keep it separate from other ingredients, wash your hands and any cutting boards or utensils that touch the raw chicken with hot, soapy water, and use a food thermometer to ensure the chicken reaches 165°F (74°C) in the thickest part before serving. Leftovers should be cooled, then stored in an airtight container in the refrigerator and eaten within 3 to 4 days; reheat gently in the oven or microwave until steaming hot all the way through.