Combine the grapeseed oil and the bonito flakes in a small saucepan and cook over low heat until the oil reaches 180 degrees F. Remove from the heat and cool at room temperature for 20 minutes before straining through a fine mesh sieve. Reserve the bonito flakes for another use (see Cook's Note).
Whisk the egg yolks together with the lemon juice, rice wine vinegar, and salt in a large bowl. Whisk briskly, then add the oil a few drops at a time until the egg mixture thickens and lightens in color a bit. Then, still whisking briskly, increase the oil flow to a constant, thin stream until a smooth mayonnaise texture is achieved.
I use these flakes on pizza, in scrambled eggs, on toast and with Manchego cheese, which sounds like it shouldn't be good but totally is.
Consumption of raw or undercooked eggs, shellfish and meat may increase the risk of foodborne illness.