Make sure egg and all ingredients are at room temperature. Take egg out of fridge and let it sit on the counter for at least twenty minutes, or fill a cup with warm water and soak for a few minutes.
Separate egg yolk.
Combine egg yolk, vinegar, lemon juice, mustard and salt and whisk vigorously. (Add spices if desired.)
SLOWLY – and I mean painstakingly slowly, drizzle in olive oil while whisking the mixture. (This can be done in a food processor.). If you add the oil too quickly the mayonnaise may ‘break’. This means the oil and the egg will separate and you will have to add mustard or more yolk and whisk vigorously to fix this blunder.
Add additional salt, seasoning or lemon juice to taste.
Store in fridge up to a week.