Preheat the oven to 375°F (190°C).
Heat vegetable oil in a large skillet over medium-high heat.
Add diced onion and cook for 3-4 minutes until softened.
Add minced garlic and cook for 1 minute until fragrant.
Add ground beef to the skillet, breaking it apart with a spatula.
Cook the beef until browned and no longer pink, about 7-8 minutes.
Drain excess fat from the skillet.
Season the beef mixture with salt, black pepper, dried thyme, and paprika. Stir to combine.
In a medium bowl, mix together the condensed cream of mushroom soup, sour cream, milk, and shredded cheddar cheese until smooth.
Add the soup mixture to the beef in the skillet and stir until well combined and heated through, about 2 minutes.
In a greased 9×13 inch casserole dish, spread half of the thawed hashbrowns evenly on the bottom.
Spread the beef and sauce mixture evenly over the hashbrowns.
Top with the remaining hashbrowns, spreading them evenly.
Sprinkle shredded mozzarella cheese evenly over the top layer of hashbrowns.
Cover the casserole dish with aluminum foil and bake for 30 minutes.
Remove the foil and bake uncovered for an additional 15 minutes, or until the cheese is melted and the hashbrowns are golden brown.
Remove from the oven and let the casserole rest for 5 minutes before serving.