Wash and blanch the spinach and drain in a colander. Wash and chop the other greens. Sauté the onions in olive oil and as soon as they are browned, add the greens. Cook for 10 minutes, then add the spinach. Season with salt and pepper and leave the mixture aside to rest. Fry the eggplant and set aside in a colander. Do the same with the courgette. In a large cooking dish, layer the ingredients thus: First sprinkle some mizithra cheese, add the eggplant, season with salt and pepper and add more cheese; then layer the courgette, season with salt and pepper and sprinkle with mizithra; top with the spinach and greens. In a pan, simmer the milk, flour and half a glass of olive oil until the mixture thickens, stirring continuously. When the bechamel is ready, layer on top of the spinach and greens. Bake the moussaka for 30 minutes at 200 οC.