Prepare vegetables: Cut the large onion into very thin slices. Cut the celery sticks into thin rings. Slice the potatoes into thin rings as well to ensure quick cooking.
Make the marinade: In a food processor, blend together garlic head, hot and cold red peppers, cumin, dried coriander, salt, vinegar, harissa, and lemon juice until smooth.
Mix marinade with vegetables: Take a portion of the marinade and mix it thoroughly with the sliced celery, onion, and potatoes.
Prepare the fish: Wash and clean the fish well, then dry it. Make small holes in the fish to allow the marinade to penetrate deeply.
Marinate the fish: Distribute the marinade inside and outside the fish, ensuring it is well coated.
Arrange for grilling: Place the marinated vegetables and potatoes in a tray, then place the fish on top. Brush the fish with a little oil.
Grill the fish: Preheat the oven or gas oven to the highest temperature. Grill the fish until it takes on a beautiful color and is cooked through.
Serve: Serve the grilled fish with the cooked vegetables and potatoes underneath. Optionally prepare Sayadeya rice as a side.