Use a ceramic, glass, or stainless-steel bowl—aluminum or copper will react with the acid and affect the color and even the flavor of the sauce.
Whisk together in a medium bowl until smooth and light:
      2 large egg yolks
      1 to 2 tablespoons fresh lemon juice or white wine vinegar
      1/4 teaspoon salt
      Pinch ground white pepper
Whisk in by drops until the mixture starts to thicken and stiffen:
      1 cup vegetable oil, at room temperature
As the sauce begins to thicken—when about one-third has been added—whisk in the oil more steadily, making sure each addition is thoroughly blended before adding the next. Should the oil stop being absorbed, whisk vigorously before adding more. Stir in:
      1 tablespoon minced scallions, shallots, or onions
      1 1/2 teaspoons minced sour gherkins or dill pickles
      1 1/2 teaspoons drained capers
      1 1/2 teaspoons drained sweet pickle relish or minced sweet pickles (optional)
      1 to 2 dashes of fresh lemon juice or hot red pepper sauce (optional)
      Up to 1 1/2 teaspoons Dijon mustard (optional)
      Salt and ground black pepper to taste
Serve immediately sprinkled with:
      1 tablespoon minced fresh parsley
      1 tablespoon finely snipped fresh chives
This sauce can be refrigerated in a covered jar for 1 to 2 days.