Preheat your oven to 400°F (200°C).
In a skillet over medium heat, brown the ground beef with the diced onion and minced garlic until the meat is fully cooked.
Thinly slice the russet potato, sweet potato, and carrot into rounds using a sharp knife or mandoline.
In a small baking dish, spread the cooked beef mixture evenly across the bottom.
Layer the sliced root vegetables on top of the beef, alternating between potato, sweet potato, and carrot.
Drizzle the olive oil over the vegetables and sprinkle with sea salt, black pepper, and dried thyme.
Cover the dish with foil and bake for 20 minutes.
Remove the foil and bake for an additional 15-20 minutes until the vegetables are soft and the top is lightly browned.