In a measuring cup, combine both grapeseed oil and olive oil.
To a blender, add the egg and egg yolk.
To the blender, add 2 Tbsp lemon juice. (This is essential, as the acid binds the ingredients.)
To the blender, add ½ tsp salt. Add 1 tsp of Dijon mustard, if using.
Blend the ingredients for a couple of seconds, and then SLOWLY drizzle the oil mixture into the blender, while blending.
That's it. Home-made Spanish mayonnaise!