Heat a saucepan to medium heat. Add 2 tbsps of olive oil and the chopped onions and sauté the onions for 3 minutes, until softened and slightly colored. Add the garlic, the mushrooms and the bacon, until most of the juices have evaporated and set aside.

For the bechamel sauce, melt 6 tbsps of butter into a deep pan, over medium heat. Add the flour and whisk to make a paste. Add warmed milk to the mixture in a steady stream, whisking continuously, in order to prevent the sauce from getting lumpy.