With your hands, flake the whitefish, being careful to discard all the skin and bones. In a medium bowl, gently combine the whitefish with the red onion, celery, mayonnaise, lemon juice, salt, and pepper. Taste for seasoning.
Toast the bagel slices in a toaster. Arrange a platter with the smoked salmon, cream cheese, tomatoes, and red onions. Serve with the whitefish salad at room temperature.