Preheat your oven to 375°F (190°C). Grease a rectangular ceramic baking dish. Arrange the thinly sliced potatoes in overlapping layers, drizzle with olive oil, and season with salt and pepper. Cover tightly with foil and bake for 20 minutes until tender.

Heat olive oil in a large skillet over medium-high heat. Add the ground beef, break it apart, and season with salt, pepper, garlic powder, and onion powder. Cook until completely browned and crumbled. Drain excess grease.

Melt butter in a medium saucepan over medium heat. Whisk in the flour and cook for 1 minute. Gradually whisk in the warm milk, stirring constantly until the mixture simmers and becomes a thick, creamy white sauce.

Remove the par-baked potatoes from the oven and take off the foil. Evenly distribute the browned ground beef over the potatoes, then pour the thick creamy white sauce over the beef. Top evenly with the grated cheddar and mozzarella cheeses.

Bake uncovered for 20-25 minutes until the cheese is bubbly. Broil on high for 2 minutes to get golden-brown charred spots. Garnish with finely chopped fresh parsley before scooping with a silver serving spoon.