1/2 cup mayonnaise
1/2 cup plain yogurt
Some Variations:Lime juice: Adds a fresh, acidic zest.
Spices: Cumin and cayenne are often included to provide earthy flavor and a touch of heat.
Herbs: Some versions include chopped fresh dill, and others incorporate cilantro. (A former Rubio's assistant manager on Reddit claimed their sauce did not contain cilantro.)
Other seasonings: Salt and pepper are used to taste.
💡I found one note where it said they added water sometimes to adjust the texture.
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Here is one of those variations, which borders on being a tartar sauce with the pickles:
1/2 cup yogurt, plain
1/2 cup mayonnaise
1/4 teaspoon cayenne
1/2 teaspoon cumin
1/2 teaspoon fresh dill, chopped or 1 teaspoon dill pickle juice
juice of 1 lime
1/2 teaspoon salt
1/2 teaspoon pepper