In a mixing bowl, flake the drained tuna with a fork until it's broken into small pieces.
Add the mayonnaise to the tuna and mix well to combine. Adjust the amount of mayonnaise according to your desired level of creaminess.
Stir in the diced celery and red onion. These add crunch and a burst of flavor to the salad.
Drizzle the freshly squeezed lemon juice over the mixture and stir to incorporate. The lemon juice adds a refreshing tang to the salad.
If desired, add chopped fresh parsley for an extra layer of freshness.
Season the tuna salad with salt and pepper to taste. Adjust the seasoning as needed.
Serve the classic tuna salad on bread for a sandwich, on a bed of greens for a salad, or as a dip with crackers or vegetable sticks.