In a large nonreactive bowl combine first 10 ingredients and season to taste with salt and pepper. Cover bowl with plastic wrap and marinate at least 8 hours or overnight, stirring once or twice.
To serve, line 6 or 8 salad plates with lettuce leaves. Arrange onion and orange slices, slightly overlapping, on each plate. Using a slotted spoon mound fish in center of plate and spoon a little liquid over top of each serving.