Drain and press the firm tofu between paper towels for about 10 minutes. Slice into 1/4-inch thick rectangles.

In a mixing bowl, combine soy sauce, sugar, minced garlic, and Korean red chili powder. Whisk until the sugar dissolves. Add water and fold in sliced green onion.

Heat a non-stick skillet over medium heat, add oil, and place tofu slices in a single layer. Cook undisturbed for about 3–4 minutes until golden brown. Flip and cook the other side for another 3–4 minutes.

Pour the sauce over the golden tofu, reduce heat to medium-low, and let it simmer for about 2–4 minutes until the sauce thickens and coats the tofu evenly.

Transfer the cooked Spicy Korean Tofu to a serving platter and garnish with reserved green onion. Serve warm or at room temperature.