Preheat oven to 350. Spread the rock salt on the bottom of each mini (this allows each onion to stand up nice and straight). Wash onions and add one to each mini. Bake for about an hour or until softened and brown. Let cool.
Slice the tops off each onion and using a spoon, remove the pulp of the onion, leaving a few layers on the outside. Be careful to not pierce the bottom of the onion. Place the empty onions back on the salt and move on to preparing the onion soup.
Chop the onion thinly and add to a skillet with butter, bay leaf and a pinch of salt. Cook for about 30 min on medium heat until caramelized. Add a spoonful of flour and cook while stirring for 1 min. Add the white wine and let the alcohol evaporate then add ½ cup of water to loosen up the onion and create a creamy soup.
Prep the béchamel by melting the butter in a saucepan and adding the flour. Cook for a couple of minutes while stirring, on a medium heat until the roux (mix of butter and flour) is golden. Then pour in the milk and bring to the boil, while stirring vigorously. Boil for 2 minutes and then remove from the heat. The béchamel should be creamy, with no lumps. Stir in the parmesan cheese, salt and pepper.
Fill the onion cavities halfway with the onion soup, place 1 egg yolk gently in each of them, season with salt and top with a few fresh truffle shavings. Then cover with bechamel and another sprinkle of cheese. Bake at 400 for about 5 min or until tops start to brown. Serve with crispy bread!
