In a large mixing bowl, whisk together gochujang, gochugaru, soy sauce, sesame oil, brown sugar, garlic, and ginger until smooth.
Add the sliced chicken and yellow onions to the bowl, tossing thoroughly to ensure all pieces are coated in the marinade.
Cover and refrigerate for a minimum of 30 minutes to allow flavors to penetrate.
Heat vegetable oil in a large skillet or wok over medium-high heat until shimmering.
Add the marinated chicken mixture to the skillet in a single layer.
Cook undisturbed for 2-3 minutes to allow caramelization, then stir-fry for an additional 5-7 minutes until chicken is fully cooked.
Remove from heat and garnish with chopped green onions and toasted sesame seeds.