Peel the onions, cut them in half and cut them in a fan shape so that the onion slices stay together. Melt butter in a Teflon pan, pour onions evenly, extinguish with wine, steam over low fire until soft, add wine if necessary. Season with salt and pepper from the mill. When the wine has evaporated, sprinkle with a little Parmesan cheese.
Brush a thin layer of Dijon mustard on the top white side of the turbot. Place a thin layer of cooked onions on top. Sprinkle the remaining Parmesan on top.
For the sauce, steam the shallots and mushrooms in butter, deglaze with Noilly Prat and champagne, boil until well reduced, add fish stock, bubble and thicken slightly with a little flour butter. Simmer gently on low heat for 15 minutes. Add cream, season with salt, pepper, a few dashes of Worchester and lemon. Strain and beat a little fresh butter. Stir in the mustard with a hand mixer, away from the fire. If necessary, add a little champagne to make it fluffy.
Steam the beet in butter, season. Turn the fish slices in a little flour on the other side and fry them with the onion side at the beginning until golden brown.
Remove the frying pan from the fire, turn the fish and cook it.
Divide the beet evenly among the plates, place the fish slices on top, pour the fluffy champagne mustard sauce over them.
By the way: Did you know that