Allow the hash browns to thaw overnight in the fridge.
Preheat your oven to 375 degrees.
Melt 3 tablespoons of butter in a skillet and add the diced yellow onion and minced garlic.
Cook the onion and garlic over medium heat, stirring often, until soft (around 7-10 minutes).
In a large bowl, mix together the hash brown potatoes, 3 cups of shredded sharp cheddar cheese, cooked garlic and onion, and the diced bell peppers.
Grease a 9 by 13 baking dish and spread the hash brown mixture out evenly.
In a bowl, whisk together the eggs, milk, salt, pepper, and Italian seasoning.
Pour the egg mixture evenly over the potato casserole.
Top with the remaining cheese and the green onions.
Bake the casserole at 375 degrees for one hour, or until the edges are browned and the cheese is bubbly