Spread the rock salt on the bottom of a baking tray and place the washed onions on top as they are. Bake at 180C for about 50 minutes or until they start to brown on the outside and get soft in the middle. Let them cool and cut them open from the top. Once opened, remove the onions’ pulp with a spoon being careful not to pierce/slash the skin. Place the empty onions back on the salt and move on to preparing the onion soup.

Chop the onion thinly and in a small pot, brown them slowly with oil, butter, a bay leaf and a pinch of salt. Cook covered with a lid for about 30 minutes on medium heat until caramelised. Now add a spoonful of plain flour and cook while stirring for 1 minute or more. Add the white wine and let the alcohol evaporate then add about 100 ml of water to loosen up the onion and bring to a gentle simmer to create a soup consistency.

To prepare the béchamel. Melt the butter in a saucepan and add in 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 whisking vigorously. Boil for 2 minutes and then remove from the heat. The béchamel should be now ready, creamy and with no lumps. Stir in the grated Grana Padano cheese, until smooth. Season carefully with salt (as the cheese will have already made the béchamel tasty) and a little black pepper. Set aside.

Now half fill the onion cavities until half 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 the Grana Padano bechamel and bake at 180 C for 5 minutes or until nicely golden on top. Serve piping hot with a few chunks of toasted bread that can be dipped into the onion to soak all of the goodness. Then dig in with a spoon as if it were a soup!