Place a large (preferably oven-proof) skillet over medium-high heat then add oil. When the oil is hot, add the hash browns. Cook without stirring for 5–7 minutes until the bottom becomes golden brown and crispy. Once crispy, transfer hash browns to a plate then set aside—don’t worry if they are still underdone on top—they’ll finish cooking in the oven.
Return skillet to the heat and brown the ground beef with the onions, salt and pepper. When the beef is cooked through and no pink remains, drain grease and return to the heat. Add milk and mushrooms then stir to combine. Scrape up any of the brown bits left on the bottom of the pan and allow the mixture to come to a bubble.
Reduce heat to medium-low, stir in spices until well combined then remove from heat. Add flour to thicken up the sauce if it's still too runny. If your skillet isn’t oven-safe, transfer the mixture to a 2-qt casserole dish. Otherwise, finish assembling the casserole in the skillet.
Sprinkle 1½ cups of shredded cheese on top of the ground beef mixture, reserving the rest to sprinkle on top. Add hash browns, browned side up, to the top of the casserole then sprinkle with remaining cheese.
Bake 18–20 minutes until piping hot. Remove from oven then allow it to cool 5 minutes before garnishing with chopped green onion.
This recipe is not complicated and, better yet, cleanup is a breeze!