A lot like mayonnaise, but generally thinner and flavoured with garlic. Best served with seafood.
Make your own by pounding 4 cloves of garlic with some sea salt using a mortar and pestle. Mix in a teaspoon of Dijon mustard and two egg yolks. Slowly add half a cup of good olive oil, mixing well as you pour it in so that the mixture emulsifies well. Add half a teaspoon of water and a full teaspoon of lemon juice, whisk well, then slowly add another half a cup of olive oil. If the mixture separates, slowly stir in another egg yolk. To this you can add other herbs and spices that you like, such as tarragon, chillies, rosemary, crushed apple, saffron, etc.
Use aïoli in these recipes: