Bring a large pot of water to a boil. Add 1 teaspoon 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, the red onion, celery, and cilantro. Gently mix to incorporate. 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.If using large shrimp (20 per pound), you'll need to cook them for a bit longer (probably 3 minutes), ad you might want to chop them into bite-size pieces, although it's fine to leave the whole.