Drain tuna into a fine-mesh strainer and press down to drain. While tuna drains stir together the mayo, Dijon, lemon juice and celery seed.
Chop the celery and capers and thinly slice the green onions.
When the tuna has drained, put it in a bowl. Stir in the dressing until it's well-distributed in the tuna, then mix in the celery, capers, and green onions.