Processed cheese and Lipton onion soup mix.
A few sources say this recipe appeared on Lipton Soup Mix boxes in 1968, and in an advertisement from Better Homes and Gardens from September of the same year.
the July 25, 1970 edition of The Calgary Herald calls for Canadian cheddar. And the ad below, from Flickr, uses Swiss for “Holiday Fondue.”
1 envelope Lipton Onion Soup Mix
2 cups tomato juice
4 tsp. lemon juice
1 lb. processed American Cheese shredded (4 cups)
French bread or rye bread cut into bite size pieces
In medium saucepan combine soup mix, tomato juice and lemon juice. Heat slowly.
When juice begins to simmer, add a small portion of the cheese and stir constantly until the cheese is melted. Continue to add cheese gradually, stirring constantly, making sure each addition is melted.
Keep warm in fondue pot or chafing dish or heated casserole. Spear cubes of bread, then diip and stir into the fondue.