In the novel Crooked House, Mario's is a restaurant. Charles Hayward sent a telegram to Sophia Leonides upon his arrival back in England, inviting her there for dinner at nine. What he didn't know was that her grandfather recently had passed away. Despite this she accepted his invitation.
The name of the restaurant is possibly a nod to Agatha Christie's friend restaurateur Mario Gallati who owned "Le Caprice" in London.