To drain the tuna, you can simply crack open the lid and drain the oil, but I prefer to use a strainer, and in this photo, a rice strainer, this ensures the best drained tuna. Once it is good and drained, add the tuna to a mixing bowl.
Slice and dice the celery, slice the spring onion, and dice the pickles. Add these to a mixing bowl. (I did not have my homemade pickles made so I added a tablespoon of chopped capers.)
To the mixing bowl add the mayo, mustard, and juice. Mix well, and season with salt and pepper as desired to taste. Start with the stated ½ cup mayo and mix, add mayo as needed to get the consistency you desire. I added about a tablespoon more.
Once you have the salad to your desired consistency, cover and place in the fridge for 1-2 hours to chill. This will store safely in the fridge about 2 days.
When you are ready for a sandwich, or two, lay out the bread of your choice, add some mayo to each slice (I used Japanese Kewpie Mayo, it is in a squeeze bottle), add 2-3 lettuce leaves to a slice of bread.
Top with ¼ of the tuna salad, top with another slice of bread, repeat for remaining sandwiches.
Cut each sandwich in half, enjoy.