Bring a large pot of water to a boil. Add 1 teaspoon of kosher salt and the shrimp. Reduce the heat to medium. Cook the shrimp, uncovered, until pink and cooked through, for about 2 minutes.
Turn the heat off and use a slotted spoon to remove the cooked shrimp onto a bowl filled with cold water to stop the cooking process. You don't want them overcooked.
Make the dressing:
In a salad bowl, whisk together the mayonnaise, Dijon mustard, vinegar, garlic, kosher salt, and black pepper.
Mix the salad:
Add the cooked shrimp, red onion, celery, and cilantro. Gently mix to combine.
Serve immediately, or refrigerate for a couple of hours. Remix the salad before serving.
Video
Notes
It's OK to use frozen and defrosted pre-cooked shrimp, but remember that they're not as good in terms of flavor and texture as freshly cooked ones.
If using extra-large shrimp (20 per pound), you'll need to cook them for a bit longer - 3 minutes. Once cooked, you can chop them into bite-size pieces, but leaving them whole is OK.
You can keep the leftovers in the fridge, in an airtight container, for up to two days. I don't recommend freezing them.