Preheat the oven to 350 degrees F. In an oven-safe skillet over medium high heat add the olive oil, onions (if using,) and bell pepper (if using.) Cook the vegetables until softened, about 5 to 6 minutes.
If you want to add precooked leftover meat add that in now to warm with the vegetables. If a thicker sauce is desired, whisk together 2 tbsp water and 1 tbsp cornstarch. Pour the cornstarch slurry into the pan. Bring the juices to a boil and cook just until thickened. Turn flame off.
Spread the meat mixture into an even layer in the cast iron skillet on the bottom. In a bowl, stir together the frozen hash browns, sour cream, half of the cheese, garlic powder, salt, and black pepper.
Place dollops of the hash brown mixture over the top, spreading if possible to even out. Top the casserole with the remaining cheese. Place the casserole into the oven and bake for 30 minutes or until the cheese has melted and the edges are bubbling.
Allow the casserole to rest for 10 minutes, then scoop out and serve.