For the mayo, place egg yolks in a mixing bowl with the lemon zest and juice, and Dijon mustard. While whisking, slowly stream in the grapeseed oil until emulsified.
In another bowl, flake tuna and combine with celery, shallots, lemon juice, 1/3 cup of the lemon mayo and black pepper. (Store remaining lemon mayo in the fridge or use for Chicken or Fish with Lemon-Parm Crust.)