Step 1 Combine the yolk, lemon juice, mustard and salt in a blender. Process the mixture briefly on low speed.
Step 2 With the motor running, very slowly dribble in the oil until the mixture emulsifies. Taste and adjust the seasoning, adding more lemon juice or salt as needed, then season with pepper. Serve immediately, or transfer to a bowl, cover and refrigerate up to three days.
Step 3Variations: For easy aioli, make mayonnaise according to recipe above but omit the salt and use really good olive oil for one-third of the oil. Use the flat side of a chef's knife to smash 4 garlic cloves to remove peels and crush flesh. Cut out any green sprouts in the garlic (they're bitter). Using a mortar and pestle, crush the garlic with 1 teaspoon coarse sea salt to make a smooth paste. Whisk into mayonnaise.
Step 4For easy rouille, make mayonnaise according to recipe above but use half olive oil, half peanut oil. Blend in 2 cloves minced garlic, 1 teaspoon cayenne and 3 teaspoons hot paprika. Let stand at least 1 hour to allow flavors to meld.