Add the salmon, celery, cucumber, radishes, cilantro, pickled red onions, scallions, capers (if using), and walnuts (if using) to a mixing bowl.
Add the thick strained kefir and lemon juice. Season with salt and pepper.
Gently mix until everything is coated. I like leaving some bigger pieces of salmon instead of breaking it up too much.
Taste and adjust the lemon juice, salt, and pepper if needed.
Serve right away, or refrigerate for 20 to 30 minutes if you want the flavors to come together a bit more.