1 large egg yolk
1 teaspoon Dijon mustard
1 tablespoon lemon juice or white vinegar
1 cup neutral oil
Pinch of salt
Optional: pinch of sugar or a little water
Add the egg yolk, mustard, lemon juice or vinegar, and salt to a bowl
Whisk until combined
Very slowly drizzle in the oil while whisking constantly
Keep whisking until the mixture thickens
Add the remaining oil in a thin stream while continuing to whisk
Adjust with a little more lemon juice, salt, or water if needed
Store in the refrigerator in a covered container