Firm monkfish fillets are simmered with orange segments, pieces of olives, and slivers of onion, which complement the low-fat fish's sweet, mild flavor.
Place onion, celery, carrot, parsley, bay leaves, peppercorns, zest, and wine in a wide shallow saucepan with 6 cups water. Bring to a boil; reduce to a simmer. Cook 20 minutes to allow flavors to blend. Strain; discard solids. Return court-bouillon to pan.
Bring court-bouillon to a boil; reduce to just simmering. Place fish in stock (it will be partly submerged); top with oranges, olives, and red onion. Cover; cook until fish is just cooked through, about 10 minutes.
Transfer fish with a spatula or slotted spoon to serving platter with oranges, olives, and onion. Serve immediately.