Preheat oven to 350°F. Grease a 9x13 baking dish.

Layer the thawed hash browns evenly on the bottom of the dish.

Sprinkle cheese and your choice of meat (or leave meat out for a vegetarian version).

Whisk together eggs, milk, salt, and ground mustard in a bowl until smooth.

Pour egg mixture evenly over the top of the potatoes and cheese.

Cover with foil and bake for 1 hour.

Uncover and bake an additional 10–15 minutes, until the top is golden and the center is set.

Let rest for 5–10 minutes before serving warm.