Preheat the oven to 350°F (175°C).
Heat vegetable oil in a large skillet over medium-high heat.
Add the chopped onion and cook for 3-4 minutes until softened.
Add the minced garlic and cook for 1 minute until fragrant.
Add the ground beef to the skillet and cook, breaking it apart with a spoon, until browned and cooked through, about 7-8 minutes.
Season the beef mixture with salt, black pepper, dried thyme, and paprika. Stir to combine.
Add the sliced mushrooms to the skillet and cook for 4-5 minutes until they release their moisture and soften.
Sprinkle the flour over the beef and mushroom mixture and stir well to coat evenly.
Gradually pour in the beef broth while stirring to avoid lumps. Bring the mixture to a simmer and cook for 3-4 minutes until the sauce thickens.
Remove the skillet from heat and stir in the sour cream until fully incorporated.
In a 9×13 inch baking dish, spread half of the thawed hashbrowns evenly on the bottom.
Pour the beef and mushroom sauce mixture over the hashbrowns and spread evenly.
Top with the remaining hashbrowns and sprinkle the shredded cheddar cheese evenly over the top.
Bake the casserole in the preheated oven for 30 minutes, or until the cheese is melted and the casserole is bubbly.
Remove from oven and let it rest for 5 minutes before serving.
Garnish with chopped fresh parsley and serve warm.