1 large egg yolk
1 teaspoon Dijon mustard
1 tablespoon lemon juice or white vinegar
1/4 teaspoon salt
1 cup neutral oil
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
Continue adding oil in a thin stream until all is incorporated
Taste and adjust salt or acid if needed
Store in an airtight container in the refrigerator