Preheat the oven to 350 degrees.
Spray the inside of a 9" x 13" baking dish with non-stick cooking spray.
In a skillet over medium heat, add the butter. Once melted, add the onion and cook, stirring frequently, until the onion lightly caramelizes - about 5 minutes. Set aside.
In a large mixing bowl, add the shredded potatoes, cream of mushroom soup, sour cream, 1 1/2 cup shredded cheese, the cooked onions, garlic salt, and chicken broth. Stir until well mixed.
Spread the potato mixture into your prepared 9"x13" baking dish and sprinkle the remaining 1/2 cup cheese evenly over the top.
Put in the oven and bake, uncovered, for 30-40 minutes or until the edges are bubbly and lightly golden brown. Remove from the oven, enjoy!