Place 1 egg yolk in a bowl
Add 1 teaspoon mustard
Add 1 tablespoon lemon juice or vinegar
Whisk until combined
Add a few drops of oil while whisking constantly
Continue adding oil very slowly in a thin stream
Keep whisking until the mixture thickens
Add salt to taste
Add more lemon juice or vinegar if needed
Whisk until smooth and creamy
Use immediately or refrigerate in a covered container