Apart from souffle one of the hardest dishes I learnt was the La Bilbaina bacalao al Pil pil.
This classic Basque Cod dish is extremely simple but the constant stirring to get the emulsion is harder than it seems and marks the foothills of Txotxo mastery.
Clue here, many recipes the trick is practice, trad way stir for 2-3 hrs or if you have the gizmo en vide see below
May I add this is the only dish I prefer to lobster or blowfish, luckily if you can find ingredients a tenth of the cost.A substitute is Hoki or Shark but nothing has the dense lush softness of Grand Banks Cod from St Peierre et Miquelon or Nova Scotia