Step 1Combine the yolk, lemon juice, mustard and salt in a blender. Process the mixture briefly on low speed.
Step 2With 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.
Sesame mayonnaise: To 1 cup mayonnaise, add 2 tablespoons Asian sesame oil, 1 teaspoon hot sesame oil, 1 to 2 teaspoons tamari or regular soy sauce, 1/4 teaspoon Chinese hot chile paste and 2 tablespoons chopped fresh chives. (Great as a sauce for asparagus.)
Chipotle mayonnaise: To 1 cup mayonnaise, add 1 finely chopped chipotle chile in adobo sauce, 1 clove minced garlic and 2 tablespoons chopped cilantro. Add adobo sauce if you want it hotter. (Serve with roast beef or grilled salmon or tuna.)
Green curry mayonnaise: To 1 cup mayonnaise, add 2 to 3 teaspoons Thai green curry paste, 1 tablespoon fresh lime juice, 1/8 teaspoon Thai fish sauce and 1 tablespoon chopped fresh cilantro or chives. (Try it as a dip for shrimp or as a dressing for coleslaw.)
Nicoise mayonnaise: To 1 cup mayonnaise, add 2 teaspoons anchovy paste, 3 tablespoons finely chopped Nicoise olives, 2 tablespoons drained and chopped capers, 1 large clove minced garlic and 2 to 3 tablespoons chopped fresh basil. (Use it in tuna salad, dolloped onto a veal chop or scallop, or as a dip for roasted potato wedges.)
Chutney mayonnaise: To 1 cup mayonnaise, add 1/2 cup Major Grey chutney (chop any large chunks of fruit), 1 tablespoon fresh lime juice and finely chopped green part of 1 green onion. (Spread it on a turkey sandwich, add it to pasta salad or mix it with an equal part grated sharp Cheddar and bake it on baguette rounds.)