Preheat the oven to 190 degrees C. Lightly grease a 20x30cm baking dish.
Heat olive oil in a frying pan over medium heat. Add celery, spring onion and garlic. Cook and stir for a few minutes until soft. Remove from heat then stir in the crab meat, bread cubes, egg, Romano cheese, lemon juice and tomato. Season with salt and 1/4 teaspoon of pepper then mix until well blended.
Lay the cod fillets in the prepared baking dish. Brush each one with melted butter. Place a heaping tablespoon of the crab mixture onto half of each fillet then fold the other half over to cover. Secure with toothpicks if desired. Sprinkle on any remaining stuffing then drizzle with any leftover melted butter. Cover the dish with a lid or aluminium foil. (The dish may be also made ahead of time and refrigerated at this point.)
Bake for 20 minutes in the preheated oven then remove the cover and bake for an additional 10 minutes, until the top has browned and the fish flakes easily with a fork.