Grill the bell peppers over Direct High heat until the skins are evenly charred on all sides, 10 to 12 minutes, turning every 3 to 4 minutes. Remove from the grill and place in a paper bag; close tightly. Let stand 10 to 15 minutes to steam off the skins. Remove the peppers from the bag and peel away the charred skins. Cut off the tops and remove the seeds. Cut the peppers into 1/4-inch strips and set aside. In a medium bowl, whisk together the remaining vinaigrette ingredients. Add the peppers and set aside for as long as 1 day.
Lightly brush or spray the fish fillets with olive oil. Season them with salt and pepper to taste. Grill over Direct High heat until the flesh is opaque in the centre, 5 to 7 minutes, turning once halfway through grilling time. Remove from the grill and serve warm with the roasted pepper vinaigrette spooned over the top.