1 large egg yolk
1 teaspoon Dijon mustard
1 tablespoon lemon juice or white vinegar
1 cup neutral oil
1/4 teaspoon salt
Optional: 1 teaspoon 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 a little water if needed to adjust the texture
Taste and add more salt or lemon juice if desired
Store in the refrigerator in a covered container