Hake in mousseline sauce

Peel and finely chop the onion.
Rinse the bouquet garni.
Melt 125 g butter in a bain-marie to clarify it (remove the whey, that is to say the white part that usually settles in the bottom).
Put the cream in the freezer.

Put the steaks in a large enough pan.
Add the chopped onions and the bouquet garni.
Wet (with water) halfway up the steaks.
Add salt and pepper.
Put over the heat and bring to a simmer for 3 min with a lid.
Stop the heat, and leave for 10 minutes in the hot broth (still with lid).

Meanwhile, in a saucepan, mix 1 tbsp. mustard with 1 tbsp. of cold water.
Add 2 egg yolks, and stir up this sabayon in a bain-marie by whisking until it becomes sparkling/foamy.
Off the heat, progressively add the warm melted butter (clarified) while beating with a whisk.
Then add the juice of 1/2 lemon. This is a "Hollandaise sauce".
Whisk aside 5 cl of liquid cream (very cold) like a Chantilly.
When it is mounted, gradually add it to the Hollandaise, it becomes the mousseline sauce.
Season to taste (salt / pepper).

Remove the steaks, and let them rest for 2 minutes in a warm spot.

Then pat them dry carefully.
Remove their central ridge; for this, plant a small, sharp knife in the heart of the ridge and carefully pull up.
Similarly, gently remove the skin by wrapping it around a fork.
Remove also any dark parts of the flesh.

Dress on warm plates, and top with mousseline sauce.

In France Hake ("Colin") is sometimes called "Merlu". If you can, get steaks from the tail (they don't show a hole in the center). But if you buy filets, then removing edges is easier!
Getting the fish cut in front of you get you a chance to look at the whole fish and check it freshness. This recipe also works with a cod or salmon steak, or a salmon fillet.
If you can replace the cooking water with a fish stock, it will be tastier. Beware, don't over cook your fish !