Preheat a waffle iron according to manufacturer's instructions; spray with cooking spray.
Mix hash browns, eggs, milk, flour, onion, sriracha sauce, garlic, baking powder, salt, and pepper together in a bowl to make batter.
Pour 1/4 of the mixture onto the preheated waffle iron; spread to the edges with a spatula. Cook until golden and warmed through, 5 to 7 minutes. Repeat with remaining batter.