3: Prepare 400 g fish fillet, remove all bones, dry on both sides with absorbing paper, then season with salt and pepper.
4: In a frying pan over medium heat, pour 3 tablespoons olive oil and when hot, place the fish fillets. Cook them on both sides, to taste, then place them on a plate. Note: It doesn't matter if they break or fall apart, we'll use them like that afterwards.
5: In the same pan, without cleaning it and still over medium heat, pour in 30 g butter and, when hot, add the shallot, salt and pepper. Cook for 1 minute, stirring, without browning.
6: Add the leeks, season with salt and mix well. Cover and cook for 3 minutes.
7: Uncover, then add 250 g liquid cream, stirring well, scraping the bottom of the pan to deglaze, and cook gently for 2 or 3 minutes.
8: Add 2 tablespoons Mustard, mix again and check seasoning.
9: Finally, add the fish and mix well.
10: Serve without waiting too long, or turn down the heat to a minimum, so as not to overcook the fish.
Remarks
You can use the fish of your choice, preferably in season. For the mustard, you can use an "old-fashioned" mustard, the seeds adding a little extra taste and texture.