Source: Niantic
A new class of Special Research, Masterwork Research is designed to take a long time to complete. The first Masterwork Research, All-in-One #151 takes over a month of daily play to complete, but the rewards are well worth the effort. Aside from Stardust, XP, and items, completing this Masterwork Research will reward Trainers with an encounter with a Shiny Mew!
Before you get too excited though, so far, the only players with access to All-in-One #151 are those who participated in the Pokémon Go Tour: Kanto event and completed the GO Tour: Kanto Research. While it may be possible that Niantic will eventually make Shiny Mew available to other players, for now, if you didn t participate in the Pokémon Go Tour: Kanto event, you cannot access this Masterwork Research.