+ MELT THE BUTTER in a small skillet over medium heat.
Add the onion and celery and cook, stirring occasionally, 5 minutes or until soft.
Add the flour, stirring to form a paste.
Add the milk and nutmeg and cook another minute or so until mixture thickens.
Meanwhile, place bread crumbs on a plate.
Scrape the mixture into a mixing bowl and add the fish, parsley, chives, salt and pepper.
Using 2 forks, mash the fish into the mixture.
Drop tablespoon dollops of the mixture into the bread crumbs.
Roll each dollop in bread crumbs, then between the palms of your hands to form balls.
Set croquettes aside on a plate while making the rest.
Fill a medium-sized cast iron or other heavy skillet with a 1 1/2-inch depth of oil and place over medium heat on the stove.
Heat the oil to frying temperature--350F.
When the oil is hot enough for frying, a droplet of water will dance across the surface of the hot oil and evaporate.
Drop croquettes carefully into the hot oil.
Fry croquettes, in batches if necessary, without crowding the skillet, until golden brown, about 3 minutes.
Using a slotted spoon, remove croquettes from the oil as they are done and drain on paper towels.
Place a linen napkin on a platter and arrange the croquettes in a pile.
Serve immediately and accompany with tomato mayonnaise.
TO FREEZE the mixture: Place croquettes on a lightly oiled cookie sheet and place in the freezer.
When the croquettes are frozen, transfer them to airtight plastic bags or a lidded freezer container.
Cook frozen uncooked croquettes directly from the freezer, increasing cooking time to 5 minutes.
TO FREEZE COOKED CROQUETTES: Cool cooked croquettes to room temperature, uncovered.
Place in an airtight plastic freezer bag or lidded container.
Label and place in freezer for up to 3 months.
To cook frozen cooked croquettes, place on a cookie sheet and place in preheated 375F oven for 20 minutes.