To prepare the rolls, open the buns and spread the butter evenly on interior of the buns. Toast the buns on a flat pan or saute pan until golden brown. For the fish, preheat the fryer to 350 degrees F with grapeseed oil.
Next pat dry the portioned fish on all sides with paper towels. Then sprinkle the fish with salt and pepper evenly. Next dip the fish in the flour and tap off the excess flour. Then dip the fish into the egg wash. Finish the fish in the panko, ensuring to coat evenly. Repeat the process for the remaining fish portions, then carefully drop the coated fish in the fryer. Cook until golden brown on both sides, 6 to 7 minutes. Remove from the fryer and place on paper towels to remove excess oil.
Season the tomatoes with salt and pepper and heat in a saute pan over medium-high heat, 1 minute per side. To build the sandwich, stack the bun with the remoulade, grilled tomatoes and caramelized onions. Finish with a portion of fish and finally top with the top half of the bun. Repeat for the remaining sandwiches.