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, about 2 minutes.
Turn the heat off and use a slotted spoon to remove the cooked shrimp to 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:
To the bowl with the dressing, add the cooked shrimp, red onion, celery, and cilantro. Gently mix to combine. Serve immediately, or refrigerate for a couple of hours. Gently mix again before serving.
It's fine 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 large shrimp (20 per pound), you'll need to cook them for a bit longer (probably 3 minutes), and you might want to chop them into bite-size pieces, although it's fine to leave them whole.