Melt the butter in an iron skillet or frying pan. Cook the onions until they become caramelized, approximately 8-10 minutes.
Add the mushrooms to the skillet and cook for 2-3 minutes.
Butter the quiche dish and add the hash browns. Pat them down and go up the side of the dish to make a crust.
Spread the mushrooms and onions over the hash browns. Sprinkle the ham, bacon and cheese evenly in the dish.
Now, whisk the eggs, milk, mustard and salt and pepper.
Pour the egg mixture evenly over the quiche ingredients.
Bake in a 350° F oven for 35-40 minutes and cook until the center is set.
Slice and enjoy!