Whisk together the mayonnaise, lemon zest and juice until combined. Add the whitefish, celery and onions, and gently mix until combined; season with salt and freshly ground black pepper. Cover and refrigerate for at least 1 hour before serving.
Serve with bagels and Pickled Red Onions.
Combine the vinegar, lime juice, sugar, salt, coriander seeds, mustard seeds and peppercorns in a small saucepan over high heat. Cook until the sugar and salt is dissolved. Remove from the heat and let cool for 5 minutes.
Put the onions in a medium bowl, pour the vinegar mixture over and toss to coat. Cover and refrigerate for at least 1 hour and up to 48 hours before serving.