With a mortar and pestle: In a large mortar, mash the garlic cloves and salt with a pestle until combined. Add the egg yolks; stir until combined. Stir in the mustard. Pour in the oil a few drops at a time, stirring until emulsified. Stir in the lemon juice. With a food processor: Process the garlic cloves, salt, egg yolks, and mustard until combined, about 5 seconds. With the machine running, pour in 1/4 cup oil in a slow, steady stream; process until the mixture is slightly thickened, about 10 seconds. With the machine still running, pour in the remaining 1/2 cup oil in a slow, steady stream; process until the mixture is thick. Stir in the lemon juice.
