Combine the brine, mustard, vinegar, garlic and salt in a tall container fit for a stick blender (immersion blender). Use the blender to roughtly blitz the garlic up.
Slowly pour in the oil and blend at the same time, to create an emulsion. It will start of thin and then will thicken. Don't pour all the oil in at once or you could run the risk of it splitting.
Taste and add more salt or vinegar if necessary.
Store in an airtight jar in the fridge for up to two weeks.