In a food processor with a whip attachment, beat the egg yolk until thick and pale in colour. Whip in the mustard powder.
While still beating drizzle in the oil, very slowly at ﬁrst until the oil has been incorporated into the egg yolk. Once the oil has blended with the egg yolk you can drizzle the oil in a more steady stream. If the mayonnaise gets to thick add some of the water.
Stir in the garlic. Blend in the vinegar and store in a covered container in the fridge for up to 1 month.