Source: BrownBudd

The sauce for Big Boy Hamburgers is a Tartar Sauce that is also used on fish sandwiches at Frisch's Big Boy in Cincinnati, Ohio.

Years before McDonald's hit the scene, there was Frisch's. Dave Frisch, who took over a well-established but conventional restaurant business from his dad in Cincinnati, Ohio met a fellow entrepreneur from Glendale, California, who had innovated a new way of cooking hamburgers - two small patties instead of one cooked faster, and made a double-decker called "Big Boy." This was in 1946. The "secret sauce" in the California Big Boy was Thousand Island salad dressing. Dave wasn't too fond of it, so he used his own sauce, which was basically a blend of mayo, catsup, and pickle relish, known as tartar sauce and served in other Frisch's restaurants with fried fish.

2 cups mayonnaise
Juice of 1/2 lemon
2 tablespoons horseradish
2 tablespoons minced onion, scallions, or shallots
2 tablespoons minced red or green pepper, sweet or hot
1/4 cup minced dill pickle/gherkins/capers
1 clove garlic, minced
2 hard cooked eggs, yolks mashed, whites chopped
1 dash Tabasco Sauce
1/3 cup catsup