Step 1 In a food processor, blend the egg, garlic and lemon zest until light and frothy, about 30 seconds. Slowly add half of the grapeseed oil until it starts to emulsify and thicken. Add the tarragon and vinegar, and season with one-half teaspoon salt and a pinch of pepper. Pulse to combine.
Step 2 While the machine is still running, drizzle in the remaining grapeseed and olive oils, then add the Parmigiano-Reggiano, blending until well combined. Adjust the seasoning. Keep in the refrigerator until ready to serve.