Place the salmon fillet in a microwave-safe dish and cover. Microwave on high for about 2 to 3 minutes, or until the salmon is opaque and flakes easily with a fork. Alternatively, use leftover cooked salmon.
Let the cooked salmon cool slightly, then flake it into bite-sized pieces with a fork.
In a mixing bowl, combine the flaked salmon, chopped celery, chopped red onion, mayonnaise, and lemon juice. Stir until evenly mixed.
Season with kosher salt and black pepper to taste, then mix again. Chill if desired before serving.
Serve the salmon salad on toast, in lettuce cups, over greens, or as desired.