1. Prepare the fish broth: put the fish carcass, water, and wine in a large pot. Bring slowly to a boil, skimming the surface. When boiling point is reached, add the other broth ingredients. Bring to a boil again. Reduce the heat and let simmer gently for 40 minutes. Filter and set aside.
2. Prepare the aïoli: whip the egg yolk and warm water in a small bowl. Add the oil drop by drop, whisking continuously, like mayonnaise. When the mixture is emulsified, add the oil more quickly, whisking constantly. Put the garlic and salt in a mortar; make a paste. Add the garlic paste and lemon juice to the mayonnaise. Set aside.
3. In a large pot, heat the oil over medium heat. Add the onion and cook until tender. Add the saffron and tomatoes, and simmer for 3 minutes, stirring. Add the filtered fish broth and simmer until the liquid is reduced to about 5 tablespoons. Add salt to taste.
4. Add the fish fillets to the broth and simmer gently until cooked, about 5 minutes. Divide the couscous among 4 hot soup bowls.
5. Place a sea bream fillet on top of each couscous portion. Dilute some harissa (to taste) in the fish broth; pour the broth over each portion.
6. Top each portion with a generous spoonful of aïoli. Garnish with chopped parsley and serve immediately.