From 1922:

Butter a soup plate well, and on it place a layer of fish, taking out the backbone; sprinkle over it some parsley and onion chopped very fine, pepper, salt, and some small pieces of butter, and then squeeze lemon juice over it. Then use another layer of fish, and season it in the same way. Place on the plate on a saucepan of boiling water, placing the saucepan lid over the plate. Steam for 15 minutes or more, according to the thickness of the fish, and serve on a hot soup plate.