Chop cucumber and celery. Finely chop red onion. Drain tuna from water (I use a small colander to do that over the sink).
Slice avocado in half, remove the seed, then slice each half into 4 sliced. Remove skin and chop.
Combine all ingredients in a salad bowl. Add olive oil, lemon juice, salt and pepper. Add parsley or cilantro (or chopped celery leaves!). Gently toss everything together. Serve immediately or cover and refrigerate for up to 3 days.
Notes
I don't like cilantro so I always use parsley. You can also use celery leaves. They are delicious and very healthy.