Place egg yolk in a bowl. Begin whisking in the grapeseed oli drop by drop, as slowly as possible. Continue in this manner until the mixture begins to thicken. Once the mayo has emulsified, you can add the rest of the oil in a slow, steady stream.
Combine diced shallot, 1 tbsp lemon juice, and 1/2 tsp salt in a small bowl and let sit for 5 minutes. Whisk in crème fraîche and buttermilk.
Gently whisk this mixture into the mayonnaise, and taste for balance and seasoning.