Separate the egg. Reserve the whites for other recipes.
Combine the egg yolk and 1 teaspoon of the acid in the bowl, whisking to combine. You can make mayonnaise in a food processor, with an immersion blender or by hand, with a mixing bowl and whisk. The key for either method is to add the oil very slowly, in a steady stream, while the processor is running or you’re whisking vigorously.
When the oil is all mixed in, the mayonnaise should be thick and fluffy, with your whisk forming ribbons through the mixture. If the mayonnaise is too thick, add enough water to thin it to the consistency you desire, adding it a teaspoon at a time.
Season with the salt and pepper and up to 1 teaspoon more acid, if desired. Store fresh mayonnaise in the refrigerator and use within 5 days. Makes 1 cup of mayo (16 1-tablespoon servings per batch).