Start by boiling your potatoes in plenty of water until they are soft.
Drain and set aside.
In the meantime, you can also prepare the "minced" vegetables.
Heat the olive oil in a non-stick pan. Add the garlic and onion and saute for 2-3 minutes. Add the mushrooms and peppers and pour the tomato sauce. Cook until your vegetables are soft.
Add salt and black pepper.
Set aside your "minced" vegetables.
To make your béchamel, mix the olive oil with the flour in a saucepan. Lower the temperature and pour in the milk. Stir until your béchamel thickens. Remove from the heat and add the grated cheese, salt, pepper and nutmeg.
Put the potatoes in your pan and mix them with the béchamel. Spread them well on the bottom of the pan. Put the "minced" vegetables on top.
Spread well in one layer and cover the entire surface of the pan with grated mozzarella. Put in a preheated oven at 180C for 50 minutes.
Let your upside-down moussaka cool for about 20 minutes before slicing.