Combine the Base: In a medium bowl or tall container, add the egg yolks, mustard, vinegar (or lemon juice), and salt. If using the immersion blender, place these ingredients together at the bottom.

Start the Emulsion: If whisking by hand, begin whisking the egg mixture, then drizzle in the oil very slowly. If using an immersion blender, immerse it into the bottom of the container, blending gently while you pour in the oil.

Continue Blending: Whisk or blend steadily until the ingredients transform into a thick, pale-yellow mixture. If it becomes too thick early, add a few drops of water or more lemon juice to loosen the consistency as needed.

Taste and Adjust: Stir in more salt, vinegar, or spices for additional flavor. You can also incorporate herbs, garlic, or chili flakes at this point to turn your basic mayo into a more flavor-packed condiment.

Store Properly: Transfer the finished mayonnaise to an airtight container or jar. Refrigerate right away, using clean utensils each time you scoop out a portion.