Peel the potatoes, cut them into equal pieces, and bring them to a boil in a pan of lightly salted water. Let them boil for 10 to 15 minutes.

Meanwhile, prepare the mustard sauce. To do this, first make a béchamel sauce. Melt the butter and let it foam in a saucepan. Whisk in the flour and continue stirring until you smell the aroma of fresh biscuits. Then add the milk, the two types of mustard, nutmeg, and salt and pepper. Cover the pan and keep warm over low heat.

Drain the potatoes, return them to the pan, and let them steam dry briefly over high heat. Shake the pan back and forth while doing so. Then turn the heat very low and add the raw endive to the potatoes, handful by handful. Stir until the endive has wilted.

Make the endive mash nice and creamy by adding some milk and cheese. Season to taste with salt and pepper.

Dust the pieces of sea bream on both sides with some flour (this prevents them from sticking to the pan). Melt some butter in a large pan and fry the sea bream on both sides until nicely browned and cooked through. Season with salt and pepper.

Spoon a mound of endive mash onto the plates, pour some mustard sauce over it, and place a piece of sea bream on top. Delicious!