Begin by preparing the octopus. Slice open the heads and remove the intestines and beaks. Place the octopus in a large mixing bowl, sprinkle with flour, and vigorously massage for about 2 minutes. This step is essential for both tenderizing and thoroughly cleansing the octopus, removing any residual grit from the suction cups. Rinse well under cold running water until the texture is no longer slippery. Drain completely.

Bring a pot of water to a rolling boil. Quickly blanch the octopus for 10 to 12 seconds, then remove and transfer to a cutting board. Allow it to cool slightly before slicing the arms and heads into bite-sized pieces. Set aside.

In a small bowl, combine the hot pepper flakes, soy sauce, sugar, potato starch, rice syrup, and ground black pepper. Mix thoroughly with a spoon until well blended, ensuring the starch dissolves evenly into the mixture.

Heat a large pan over high heat and add vegetable oil. Once the oil is shimmering, toss in the garlic, ginger, chili pepper, onion, and green onion. Stir-fry for about 3 minutes, allowing the garlic to turn golden and the onions to soften and become translucent.

Add the prepared octopus along with the seasoning sauce. Stir everything together for 2 to 3 minutes, ensuring each piece is evenly coated and absorbs the bold, spicy flavors. Remove from heat and drizzle in the toasted sesame oil, mixing well.

Transfer the stir-fried octopus to a serving plate and serve immediately with a bowl of warm rice.