Preheat oven to 375°F (190°C).
In a 12-inch oven-safe skillet over medium-high heat, cook ground beef until browned. Drain excess grease.
Add diced onion to the skillet and sauté for 3-4 minutes until translucent. Add minced garlic and cook for 1 minute.
Push meat mixture to the sides of the skillet. Melt butter in the center, then whisk in flour for 1 minute to form a roux.
Slowly whisk in beef broth and heavy cream. Stir constantly until the sauce thickens and begins to simmer.
Season with salt, pepper, and thyme. Remove from heat and stir in 1 cup of the cheddar cheese.
Gently fold in the potato slices, ensuring they are coated in the sauce and distributed evenly.
Cover the skillet with aluminum foil and bake for 30 minutes.
Remove foil, sprinkle with the remaining 1 cup of cheese, and bake uncovered for an additional 15 minutes or until potatoes are fork-tender and the top is golden.
Let rest for 5 minutes and garnish with fresh parsley before serving.