Rinse the fish and pat it dry. Rub 1 teaspoon of salt all over. Pour half of the olive oil into an ovenproof dish. Turn the fish in the oil to coat it. Refrigerate and marinate for 15 minutes.
Heat the oven to 200°C (392°F).
Peel the onions and finely chop them. Cut the green pepper in half, remove the stem, ribs, and seeds. Wash and dry the pepper, then cut it into small pieces.
Heat 2 tablespoons of oil in a pan over medium-high heat. Add the chopped onions and diced green pepper. Fry for 3-4 minutes until the vegetables soften. Roughly chop the walnuts and add them to the pan. Fry for 5 minutes, stirring occasionally. Stir in 2 tablespoons of chopped parsley and 2 tablespoons of pomegranate seeds (or 100 g pitted grapes if using instead). Season with 1 teaspoon of salt and 0.5 teaspoon of black pepper.
Remove the fish from the marinade. Stuff the cavity with the filling. Secure the opening with wooden skewers. Bake in the oven at 200°C (392°F) for 50 minutes.
While the fish bakes, peel and crush the 3 garlic cloves. In a bowl, mix the crushed garlic with the sesame paste, the remaining olive oil, 4 tablespoons of water, the lemon juice, and the remaining salt. Stir until smooth.
When the fish is done, remove it from the oven. Immediately sprinkle the remaining pomegranate seeds (or remaining grapes if you used them) and the remaining chopped parsley over the fish—this prevents them from burning.
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