For the béchamel:
Preheat the oven to 180 ° C. Melt the butter in a pan and cook the onions until they are softened. Add the garlic and cook for a few more minutes. Add the Portobello mushrooms, the porcini after you drain them and thyme. Cook for 15 minutes or until softened. Add salt and pepper according to your preferences.
For the béchamel, melt the butter in a saucepan and gradually add the flour while stirring well. Allow the mixture to saute for a few minutes and then add the milk, by stirring continuously until it thickens. Add salt and pepper. Finally, add the Parmesan.
In a pyrex pan, apply the 1/3 of the mushrooms, prosciutto and mozzarella. Lay on them a layer of lasagna and 1/3 of the béchamel. Repeat, and finish with a layer of béchamel sauce. Apply panko and Parmesan on top and bake in the oven for 45 minutes or until golden.