Preheat the oven to 400° Fahrenheit (200° Celsius).

Grease a 9x13 inch baking dish with olive oil: pour some in the middle of the dish and tilt the dish from side to side to ensure the olive oil spreads evenly across.

Drizzle 2 tablespoons of olive oil in a large pan. Put the pan on the stove and heat it over medium heat.

Add the onions and cook, stirring frequently, until the onions are lightly browned and sizzling.

Add the ground beef, breaking it into even pieces with a spoon or spatula. Season the meat with salt and pepper and cook until browned. Remove from the heat, drain, and set aside.

Peel the potatoes and cut them into ¼ inch thick slices.

Spread two layers of potatoes across the bottom of the baking dish and season with salt and pepper.

Cover the potatoes with the cooked ground beef, spreading with a spoon to create an even layer. Season again with salt and pepper.

Top with the remaining two layers of potatoes.

Set aside the prepared moussaka while making the topping.

Whisk together the eggs, yogurt or sour cream, milk, salt, and pepper in a medium bowl until fully incorporated.

Pour the prepared sauce over the baking dish with the layered potatoes and ground beef.

Bake the moussaka until the potatoes are tender and just golden brown on the top, about 45 minutes-1 hour.

Let the moussaka settle for 10-20 minutes before serving.