The fact that this dish is called a tajin means only that it is cooked in the earthenware casserole of the same name. Though nearly all tajins in Tunisia are cooked with eggs, this one is not. Spoon the remaining sauce in the casserole over the cooked fish balls when serving.
Spicy Ragout of Fish Balls (Tajin bil-Hut)
Soak bread in water and dry by squeezing by hand as if making snowball. Process egg in food processor 4 seconds. Add bread, fish, parsley, 3 garlic cloves, onion, 1 tablespoon harisa, 1 1/2 teaspoons salt and 1 teaspoon pepper and process until pasty, scrapping down sides if necessary. If your food processor isn’t big enough, you may have to do this in batches. Remove paste from processor. Using wet hands to keep paste from sticking, roll into walnut-size balls. Flatten balls slightly so they look more like small hockey puck (or falafel).
Heat oil in skillet over medium-high heat until smoking, then cook several fish balls until light golden brown, about 1 to 2 minutes, turning once. Do not crowd skillet as you cook them. Fish balls should be only half cooked. Repeat until all fish balls are browned.
In casserole (preferably earthenware), bring to boil 1/4 cup oil used for cooking fish, water, remaining garlic, tomato paste, remaining harisa, 1/2 to 1 teaspoon salt, and remaining pepper. Add all browned fish balls, reduce heat to low and simmer until sauce is reduced by half and fish balls are springy to touch, about 20 minutes.