Slice the pork, onion, and cucumber thinly.
In a bowl, mix gochujang (Korean chili paste), soy sauce, sugar, vinegar, minced garlic, and sesame oil to make the sauce.
Heat oil in a pan and stir-fry the prepared vegetables and pork.
Add the sauce and stir-fry for another 3-4 minutes until well combined.
Transfer to a bowl and sprinkle with sesame seeds to finish.