For the pickled red onions: Thinly slice the red onion, then pack into a 16-ounce mason jar. Add the pickling spice. Heat the vinegar, salt, sugar and 1 cup water in a medium saucepan over medium heat. Stir until the sugar and salt dissolve, about 1 minute. Cool about 5 minutes, then pour over the onions. Set aside to cool to room temperature for at least 1 hour, then refrigerate until ready to use.
For the garlic aioli: Combine the mayonnaise, lemon juice, garlic salt, salt and pepper in a small bowl. Cover and refrigerate until ready to use.
For the sandwiches: Preheat the grill to medium-high heat.
Mix together the garlic powder, paprika and a pinch of salt and pepper in small bowl. Coat the fish evenly with the oil and sprinkle the seasoning mixture on both sides.
Spray the grill grates with cooking spray, then grill the fish until just cooked through, 3 minutes on each side. Just before the fish is finished, add the rolls to the grill and toast for 1 minute. (See Cook's Note.)
Assemble the sandwiches with the grilled ciabatta, fish, garlic aioli, pickled onions and arugula.
For an extra pop of flavor on your fish sandwich, brush the fish and the buns with the garlic oil we made for the Grilled Potatoes.