In a large skillet heat 3 tablespoons of the oil over moderately high heat until it is hot, add the fillets, skin side down, and cook them for 4 minutes, or until the skin is very crisp. Turn the fillets and cook them for 2 minutes more.
In another large skillet cook the onion in the remaining oil over moderately high heat, stirring, for 2 minutes. Add the fresh coriander, coriander seeds, saffron, green peppercorns, orange zest, garlic and bell peppers and cook the mixture, stirring occasionally, for 7 minutes. Add the wine and the orange and lemon juice and tomatoes and cook the mixture with the salt and pepper.
Transfer the fish to a large glass or ceramic dish, cover it with the vegetable mixture and chill it, covered, overnight.
Before serving, let the fish stand in a cool place for 1 1/2 hours. Garnish each serving with two of the steamed mussels, if desired.
Recipe courtesy of Guy Reuge of Mirabelle Restaurant, St. John'sLong Island