After slicing the eggplants, arrange rounds in a colander or on a paper towel. Sprinkle with salt, allowing rounds to weep for 20 minutes. Dry, then fry the eggplant and potatoes in a quarter-inch of olive oil. Set aside on a paper towel or brown paper bag. Preheat oven to 350°.
In a large frying pan, melt 4 tbsp butter over medium heat. Add onions and sauté until softened. Add ground meat and cook until well-browned. Add tomato paste and wine, tasting for seasoning. When the paste has darkened and the moisture absorbed, remove from heat and stir in grated cheese.
Melt 2 tbsp of butter in a small saucepan. Add flour, stirring and cooking until thickened and browned to form a roux. Slowly add milk and stir until warmed but not thickened. In a small bowl, whisk eggs with nutmeg. Add to white sauce, then taste for seasoning.
Butter a 9x13-inch pan. Add a layer of eggplant, then all the potato slices. Cover with meat sauce. Add a second layer of eggplant, then top with white sauce. Bake for 45 minutes, or until the top is bubbly. The moussaka can also be assembled in advance, stored in the fridge overnight, and cooked the following day.