This is the perfect copycat In N Out sauce that tastes exactly like the spread you get at the restaurant. After a lot of research (and even calling to verify ingredients) I'm excited to share the sauce recipe with you! This recipe makes about 1 cup of sauce.
In a small mixing bowl, mix together distilled white vinegar, sugar, and ground mustard. Mix well until the sugar, mustard, and salt have dissolved.
To the same bowl, add mayonnaise, ketchup, and relish. Mix together and let sit for 10 minutes to allow the flavors to meld. Store in the fridge for up to 1 week.
Notes
You'll notice that I recommend whisking the sugar, ground mustard and salt into the vinegear until dissolved first - this ensures they are properly dispersed throughout the mayonnaise when mixing. These are the most suseptible to clumping, so I like to dissolve them into the vinegar first.
Sauces always taste better as the flavors meld together! Let this sit for at least 10 minutes but as long as possible (covered, in the refrigerator) for the best results.
You can store this in an airtight container in the refrigerator for up to 1 week. Do not leave it outside in room temperature - it will spoil!
Make sure to use a clean utensil every time to avoid contamination.