Preheat oven to 375°F and lightly grease a 9x13-inch baking dish with olive oil.
In a large bowl, toss potatoes, onion, olive oil, garlic powder, 1/2 teaspoon salt, and pepper until evenly coated.
Spread mixture evenly in the baking dish and bake for 20 minutes until nearly tender.
In another bowl, whisk eggs until frothy, then mix in sour cream, milk, garlic, and remaining salt.
Pour egg mixture over the potatoes evenly.
Bake the Easy Spanish Tortilla Casserole for 35 minutes until golden and set.
Cool slightly, garnish with chives, and serve warm or at room temperature.