Combine the grapeseed oil with 1/3 cup of the olive oil. In a small bowl, pound the garlic cloves and salt together with a pestle, then scrape the mixture into a large bowl. Beat in the egg yolks. This can be done using an electric mixer or by hand with a whisk or hand-held beater. Very, very slowly drizzle in about ½ teaspoon of the oil mixture at a time, gently whisking it into the egg yolks. Repeat this until a thick emulsion has formed.
Once the emulsion has formed, continuing whisking in the oil, about 1 teaspoon at a time, until the mixture is thick and the oil has been incorporated. If the emulsion will hold it, whisk in more of the olive oil. Gently stir in the garlic and salt mixture. Season with a little pepper. Cover and refrigerate until serving.
Yield Makes about 3/4 cup (6 fl oz/180 ml)