In a medium bowl, whisk together all ingredients until well incorporated.
Mayonnaise: Use only full fat for the best flavor. I use Best Foods.
Ketchup: I like a sweeter, less vinegary ketchup. I use Heinz. According to a TOH taste test, it's the best to use.
Granulated Sugar: Don't skip it. It's one of the main ingredients that makes this taste like the best restaurant's special sauces.
Garlic Powder and Onion Powder: We only use small amounts of these two ingredients, but, they add nice layers of flavor.
Double the recipe! This recipe makes about 8 ounces of sauce. In my house, that will generously dress 6 burgers. It never seems to be enough at our house. I generally double the recipe so the fam can not only use it as a spread on the burger but they can also have it on the side to dip fries in too.
Make it Spicy: Spicy burger sauce is easy to make with this recipe. My favorite ways to do so is either add your favorite hot sauce (to taste) when you add the ketchup, or stir in chopped fresh or pickled jalapenos or add a few teaspoons of adobo sauce from canned chipotle chiles in adobo.
Make It Like Smash Burger Sauce: Skip the ketchup and use French's prepared mustard (maybe with a little Dijon mustard as well) instead of the ketchup. Then swap the sweet relish and add chopped dill pickles along with a couple of teaspoons of the pickle juice. Lastly, omit the sugar.