In a mixing bowl, combine the coleslaw mix, radishes, and red onion. Add just enough of the marinade to moisten the coleslaw, about 2-3 tablespoons. Season with ½ teaspoon salt and ¼ teaspoon pepper and let sit at room temperature while the fish marinates.
Season the fish with salt and pepper. Pour the remaining marinade over the fish in a food storage bag and let sit at room temperature for 15-20 minutes.