Bring a small pot of water to boil and blanch the celery stalk (leave leaves) for less than a minute.
Brush one tablespoon of olive oil over a grilling pan. Heat the pan a few minutes and grill the swordfish fillets for about 2 minutes per side.
Heat the remaining olive oil in a large skillet, add and brown the onion.
Add the celery, olives, and capers and, sauté for a few minutes.
Add the tomatoes and swordfish fillets, reduce to low heat, cover, and cook for 5 minutes. If needed, add a little water.