Béchamel of onions and dried mushrooms

Béchamel, the most versatile of cooking sauces, is always a great way to delicately flavour the finest dishes. To give it a more original flavour, in this recipe we offer it with the addition of two dried ingredients: onions and mushrooms.
L'drying of mushrooms is now well known (given the section we are in!), but for the onions we can make a quick review: hull them and cut them thinly into rounds or cubes of no more than 3 to 5 mm on a side, then place them on racks and dry them at 50° until they are crispy (it can take 12 to 24 hours, turn them over halfway through the cycle).

Ingredients
500 ml boiling water
250 g dried onion
65 g dried mushrooms
60 g butter
2 tablespoons of flour
25 ml milk
half a teaspoon of salt
125 g dried onion
2 tablespoons of cheese

Procedure
We pour boiling water over the 250 g onion and the dried mushrooms and leave soaking for 45 minutes.
In the meantime we do melt half of the butter in a saucepan and then add flour and then the flush milk. We then add the salt and cook, stirring until we obtain a homogeneous and dense mixture.
Now add the onions and the rehydrated mushrooms and well drained and pour everything into a baking dish. Melt the rest of the butter and add the 125 g of unrehydrated dried onions and the cheese to the baking dish (on top of the mixture). We bake at 180° until the béchamel simmers and lightly browned (it will take about 20-25 minutes).