Easy Cheesy Sausage Potato Breakfast Casserole



1 pound of sausage (mild, all-natural, hot and spicy or chorizo would work)
1 medium yellow onion, chopped
1 tablespoon butter
3 cloves of garlic, minced
1 teaspoon salt
½ teaspoon ground black pepper
1 20-oz bad of frozen potato cubes, thawed
5 eggs
½ cup milk
2 cups shredded cheddar cheese
2 tablespoons fresh chives
*optional: ⅓ cup sour cream