Begin by making the onion relish. Heat the oil in a large frying pan over medium-high heat. Add the onion and cook for 5 minutes, or until lightly brown. Stir in the sugar then reduce the heat to low and continue to cook for 5 minutes more.
Whisk the flour and paprika in a shallow bowl. Add the oysters and mix them around so they are well-coated in flour.
¼ cup all-purpose flour, ½ teaspoon sweet paprika, 12 oysters
Whisk the egg in a shallow bowl. Add the oysters to the bowl (discard any remaining four) and mix so they are coated in egg.
1 large egg
Add the panko to another shallow bowl. Remove one oyster from the egg bowl and press it into the panko. Repeat with the remaining oysters.
½ cup panko
Heat 2 inches of oil in a high-sided frying pan over medium-high heat. Add the oysters and cook until they are golden brown on both sides, about 3 minutes. Remove them from the oil with a slotted spoon and place them on a paper towel-lined plate.
Oil for frying
To prepare the sandwich, spread the 2 tablespoons of chipotle aioli on each of the buns, top with onion, fried oysters, and slaw.