Preheat the oven to 350 degrees F. Grease an 8-inch square pan and add the thawed hash brown potatoes, arranging them in an even layer.

In a large skillet, place the sausage and diced onion. Cook and stir over medium heat until the sausage is browned and crumbled. This should take about 10 minutes. Afterwards, drain the mixture.

In the meantime, whisk together the eggs, milk, onion powder, garlic powder, salt, and pepper in a large bowl until well combined. Pour this mixture over the potatoes and sprinkle half of the Cheddar cheese on top.

Add the sausage mixture on top and sprinkle with the remaining Cheddar. Cover the dish with aluminum foil.

Bake the casserole in the preheated oven for 1 hour. Remove the foil and return the casserole to the oven. Bake until a knife inserted into the center comes out clean, which should take about 10 minutes. Let the casserole stand for 5 minutes before serving. Enjoy your Hash Browns & Eggs Breakfast Casserole!