Preheat a broiler.

Brush both sides of bread lightly with garlic butter and cut into 1/4-inch cubes. Place bread cubes in a 4-inch nonstick ovenproof skillet over medium-low heat until toasted.

Meanwhile, in a small bowl, whisk together egg, milk, parsley, and salt; season with pepper. When bread cubes begin to turn golden and garlic is fragrant, add egg mixture to skillet. Using a heatproof rubber spatula, gently pull the cooked eggs from the side of the skillet, tilting pan to let the raw egg take its place; cook until no raw egg remains.

Transfer to broiler and cook until egg is set, but still glossy, 10 to 15 seconds. Serve immediately, topped with scallion slices.