Finely mince your white onion. You want very small pieces so the flavor spreads evenly. If the pieces are too big, the texture will be chunky. Use a sharp knife for the best results.

Scoop one cup of mayonnaise into your mixing bowl. Use a full-fat version for the creamiest results. Smooth it out with your spoon to remove any air pockets. This is the heart of the sauce.

Add the sweet pickle relish to the mayonnaise. Make sure to include a little bit of the juice. This adds sweetness and a tiny bit of vinegar. It gives the sauce that classic green speckled look.

Stir in the yellow mustard. This provides the bright yellow color and a sharp tang. It helps balance the sweetness of the relish. Mix until the color is a pale, creamy yellow.

Add the dried tarragon, sugar, and black pepper. Crush the tarragon between your fingers as you drop it in. This releases the oils and makes it smell amazing. The sugar rounds out the sharp edges.

Whisk everything together vigorously. You want a uniform texture throughout the entire bowl. Ensure the spices are not clumped in one spot. Taste a small amount to check the balance of flavors.

Transfer the sauce to a glass jar. Place it in the refrigerator for at least thirty minutes. Chilling allows the dried herbs to soften. The flavors will meld together and taste much deeper.