Prep the lemon and the garlic - Wash and dry then lemon, then cut off and discard the ends. Cut the lemon into small pieces; remove and discard seeds. Peel and smash the garlic.
Combine the ingredients - Transfer lemon to a blender, and process until finely chopped, about 1 minute. Add oil, vinegar, mustard, garlic, salt and pepper; process until smooth and emulsified.