1 large egg yolk
1 teaspoon Dijon mustard
1 tablespoon lemon juice or white vinegar
1 cup neutral oil
Salt to taste
Whisk the egg yolk, mustard, and lemon juice or vinegar together
Slowly add the oil in a thin stream while whisking constantly
Keep whisking until the mixture thickens and emulsifies
Season with salt
Store in the refrigerator in a sealed container