Break the egg into a pint wide-mouth jar. Add the salt, mustard and lemon juice (and the pepper and garlic if using). Pour the oil on top but do not stir.
Immerse a stick blender to the bottom of the jar. Hold on the bottom and blend 5-6 seconds until the bottom starts to look creamy and whitish. Move the blender up and down slowly until the mayonnaise has emulsified. Alternatively, a blender will work in place of a stick mixer: blend all ingredients except the oil and gradually add the oil while blending continuously.
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