Boil spinach for one minute, drain and reserve the liquid.
Put 450 ml of the water into a pan and add the lentils.
Cover and cook over a medium-low heat for 20 minutes.
Meanwhile, heat 1 tbs of oil in a large frying pan and cook the onion and garlic for about 10 minutes until soft and golden.
Stir the mushrooms into the onions, increase the heat and season generously.
Cook until juicy and darkened in colour.
Add the tomatoes to the pan and cook briskly for 5 minutes until thickened but still quite liquid.
Add the lentils and any cooking liquid, 1/2 teaspoon salt and 1 teaspoon cumin.
Simmer gently for a few minutes.
Squeeze all the liquid out of the spinach and stir it into the pan.
Add a little water if the stew seems too dry.
Boil the potatoes in salted water, drain, cool slightly and slice lengthways.
Pre-heat the oven to 400 F or 200 degrees C.
Trim the eggplant and slice lengthways about 1/2 cm thick.
Smear a baking sheet with remaining oil, lay out the slices.
Bake for 15 minutes, turn and cook the other side for a further 5 minutes until soft.
Reduce the temperature to 360 F or 180 degrees C.
Take a 2-litre capacity dish at least 5 cm deep and make a layer with half the potatoes, then half the eggplant, followed by the mushroom mixture, the rest of the eggplant and the rest of the potatoes.
To make the topping, melt the butter in a saucepan, stir in the flour and gradually add the milk and bring it to the boil.
Simmer for a few minutes, add the parmesan and the ricotta mixed with the egg.
Remove from the heat, taste and adjust the seasoning.
Pour the sauce over the moussaka and dust with nutmeg.
Bake for 30 minutes or until the top is set and golden.