Preheat the oven to 180˚C. Whisk the egg yolks with a whisk and mustard. Stir in the olive oil until smooth.
Preheat the oven to 180°C. Pour 2tbsp of olive oil into a pan and fry the onion and garlic until the onion is soft.
In a large bowl, mix together the flour, powdered sugar and salt. Dice the butter on a cutting board. Pour the flour on top and use a knife to cut it into fine crumbs.
Peel the zest off the lemon. Divide the lemon into slices, peeling and removing the white membranes.
Put all the ingredients for the sourdough in a bowl and mix together. Cover the bowl with cling film and leave to rise in a warm place (25-26°C) for 4 hours.
In a large bowl, mix together the leaven and eggs. Add flour, sugar and salt. Knead dough for about 7 minutes with a food processor or with your hands until smooth.