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 into 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 pre-cooked shrimp, but choose a high-quality brand, thaw them for 24 hours in the fridge, and drain them well on paper towels to prevent them from being mushy. I like the 365 brand pre-cooked shrimp.
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-sized pieces, but leaving them whole is OK, too.
Although it's fine to serve this salad immediately, it improves after 30-60 minutes in the fridge and another gentle stir. As it rests, the flavors meld and the onions macerate, becoming softer and milder.
You can keep the leftovers in an airtight container in the fridge for up to three days. I don't recommend freezing them.