The Proof is in the Patisserie - Holmes & Watson, #13

Celebrity chef Hans Guggenheim has been complaining to the London Metropolitan police that he's been receiving online threats, and the exceedingly busy Detective Inspector Virginie Garrigues is forced to allocate this case to her underlings. But dealing with egotistical big-name celebrities, their staff, under-appreciated female chefs, and a story of stolen culinary techniques, is all a little beyond the skillset of the young detectives. Luckily, their boss remembers Herc Poirot. Once a dessert chef, Poirot now runs a tea import business from his home in Vauxhall, assisted by his judgemental assistant, Miss Lemon. Having helped Holmes & Watson solve the random assaults by nefarious tattoo artists in "The Vanity of Men", Herc Poirot comes into his own in this short story, as he moves from supporting the police, to working undercover at La Patissere Breton, whose chef-owner wants help finding who gave the secret of her award-winning dessert to the rapacious Hans Guggenheim.
 This delicious short story, as part of the wider Holmes & Watson story, introduces Herc as an investigator in his own right, and asks those questions; How do the Hardly Boys know who Mata Hari is, and what on earth is an éclair des ottomans?