Preheat your oven to 375°F (190°C). Lightly grease a 9x13-inch baking dish with nonstick spray or a thin layer of butter to prevent sticking.

Heat the olive oil in a large skillet over medium heat. Add the diced onion and sauté for about 4–5 minutes, until translucent and soft. Add the garlic and cook for another 30 seconds until fragrant. Crumble in the ground beef and cook until browned and no longer pink, about 6–7 minutes. Season with salt, pepper, paprika, thyme, and red pepper flakes. Remove from heat and set aside.

In a medium saucepan over medium heat, melt the butter. Once melted, whisk in the flour and cook for 1–2 minutes to form a roux. Gradually whisk in the milk and heavy cream, stirring constantly to avoid lumps. Cook for 3–5 minutes, until the sauce thickens and can coat the back of a spoon. Reduce heat to low and stir in 1 cup of the cheddar cheese and all of the Parmesan. Stir until melted and smooth. Remove from heat.

Layer one-third of the sliced potatoes on the bottom of the prepared baking dish, slightly overlapping. Top with half of the ground beef mixture. Pour one-third of the cheese sauce over the beef and spread evenly. Repeat with another layer of potatoes, the remaining ground beef, and another third of the sauce. Finish with the final layer of potatoes and pour the remaining sauce on top. Sprinkle with the remaining ½ cup of cheddar cheese and all the mozzarella.

Cover the casserole tightly with foil and bake for 45 minutes. Remove the foil and continue baking for another 25–30 minutes, or until the potatoes are tender when pierced with a knife and the top is golden and bubbling.

Allow the casserole to rest for 10–15 minutes before slicing. This helps it set and makes serving easier. Garnish with chopped fresh parsley, if desired, and serve warm.