1 egg yolk
1 teaspoon Dijon mustard
1 tablespoon lemon juice or vinegar
1 cup neutral oil
Salt to taste
Whisk the egg yolk, mustard, and lemon juice or vinegar in a bowl
Slowly add the oil a few drops at a time while whisking constantly
Continue adding the oil in a thin stream until the mixture thickens
Season with salt
Adjust with more lemon juice or vinegar if needed