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Drain the can of tuna. Optionally, rinse it under cold water to remove most of the salt, as well as the oil if oil-packed. Empty the can into a large bowl.  If you're using a more expensive can of tuna packed in olive oil, skip this step. Use the oil it was packed in instead of the mayonnaise. Empty oil in the trash can, not the sink. Add the mayonnaise, mustard, lemon juice, chopped celery, and pepper. Stir thoroughly to combine. Lightly toast two pieces of bread, then spoon the tuna mixture between them. Fresh-baked, crusty bread such as ciabatta or sourdough delivers the best taste. Balance the sandwich with arugula, lettuce or cucumber for a fresh taste. . You can add many other ingredients to your tuna mixture, such as a small spoonful of capers or a dash of hot sauce. Turn it into a tuna melt by adding a slice of cheese and heating the sandwich until crisp with a frying pan, broiler, or toaster oven. Check out even more variations here. When making a tuna melt, add the arugula or other fresh veggies after the sandwich is toasted.

Summary:
Drain the tuna. Mix in other ingredients. Spread over toasted bread. Add vegetables. Try other variations