Creamy Mushroom Gravy Scalloped Potatoes

Name: Creamy Mushroom Gravy Scalloped Potatoes
Ingredients: 3 pounds Russet or Gold Potatoes (, peeled and sliced thin); 1 medium Onion ((sweet onion is best!), sliced thin); 2 cups shredded Cheese ((cheddar, parmesan or goat cheese) , divided); 1/2 cup Butter (, divided (one stick)); 16 ounces Mushrooms (, sliced); 3 cloves Garlic (, minced or crushed); 1/3 cup Flour; 1 cup Broth ((beef, chicken or vegetable)); 2 cups Milk; Kosher Salt (, to taste); fresh cracked Black Pepper (, to taste); Minced Parsley (, optional garnish)