Combine grapeseed and olive oils in a glass measuring cup with a spout.

In a blender or food processor, combine egg yolks, mustard, lemon juice and salt to taste. While the blender is running, add a few drops of the oil and blend until combined. Continue adding the oil, a few drops at a time, until the mixture is slightly thickened and pale yellow in color. Slowly add the remaining oil in a steady stream, blending continuously until creamy.

Thin the mayonnaise to your liking with the boiling water. Taste and season with salt and lemon juice as needed.

Refrigerate until ready to use.