Thaw the frozen spinach in the microwave. Pour off the excess liquid.
Under your palm, swirl the lime a couple of times against the table to make it softer. Cut the lime in half and squeeze the juice into a bowl. Stir in the mayonnaise (leave a little mayonnaise on the tops of the rolls). Add the spinach as well.
Fry the fish fingers or fillets until crispy in butter. Warm the rolls in a toaster oven.
Pile the burgers so that the bottom of the bun is covered with the lime and spinach mayonnaise, the fried fish on top and then the mayonnaise smeared on the burger lid.