Pat 510g of firm tofu very dry and cut into slices about ¼in / 5mm thick.
Mix 1 tablespoon of soy sauce, 1 teaspoon of kosher salt, 1 teaspoon of sugar, 1 tablespoon of gochugaru, and ½ cup of water.
Heat 3 tablespoons of neutral oil in a nonstick skillet over high heat. Fry the tofu for 7-8 minutes per side, until golden and crisp. Transfer to a plate, keeping the oil.
Chop 2 scallions, reserving a little for garnish. Fry the rest with ½ cup of minced onion and 1 garlic clove, minced, in the remaining oil over medium-high heat until lightly browned.
Add the sauce and simmer until slightly reduced. Return the tofu and cook for 1 minute, turning gently to coat.
Stir in 1 teaspoon of toasted sesame oil. Scatter with 1 teaspoon of toasted sesame seeds and the reserved scallion.
