Put in bowl or food processor. Add the egg, mustard and 1tbsp of vinegar. Whisk for 2 minutes.
Put both oils in a pouring jug. Very slowly add the oil, whisking continuously. Once it thickens you can start to add the oil a bit quicker. This step will take about 5 minutes.
When all the oil has been added, add the lemon juice and remaining vinegar and blend.