Lightly butter a shallow baking dish and place in it the carrots, leeks, butter, salt, and pepper.
Cover with a piece of foil.
Place in a 350° oven and cook for 20-25 minutes, stirring frequently.
Lay filets on a work surface with the skin side down.
Place a spoonful of vegetables in the center of each filet and fold over lengthwise.
Fold the ends under to keep the vegetables inside.
Place remaining vegetables in a layer on bottom of a steamer basket.
Lay filled filets on top of vegetables.
Pour fish stock (reserving 1/2 cup for sauce) in bottom of steamer.
Place the steamer basket containing fish and vegetables in the steamer.
Cover and steam 8-10 minutes, until just done.