Cut fish into tiny pieces, or coarsely chop until it holds together.

Mix with eggs, 1/3 c. mayonnaise, breadcrumbs, chives, 1 tsp. salt and cayenne pepper.

Form into 8 patties. Refrigerate, covered, at least 1 hour and up to 8 hours.

Heat 1 T. oil in a large, heavy skillet over medium heat. Cook four burgers, flipping once, until golden brown and cooked through, about 5 minutes per side.

Repeat with remaining oil and burgers.

Combine remaining 1/2 c. mayonnaise and mustard. Lightly toast buns in skillet.

Sandwich burgers in buns with mayonnaise mixture, sweet onions and lettuce.