In a medium bowl, whisk together the soy sauce, brown sugar, honey, minced garlic, black pepper, and red pepper flakes (if using). Add the steak cubes to the marinade, ensuring they are well coated. Cover and refrigerate for at least 30 minutes, or up to 2 hours for more flavor.
Heat olive oil in a large skillet over medium-high heat. Once hot, remove the steak from the marinade, allowing excess marinade to drip off. Reserve the marinade for later.
Add the steak cubes to the skillet in a single layer. Cook for about 3-4 minutes, turning occasionally, until browned on all sides and cooked to your desired doneness.
Pour the reserved marinade into the skillet with the cooked steak. Bring to a simmer and cook for an additional 2-3 minutes, allowing the sauce to thicken and coat the steak bites.
Remove from heat and sprinkle with sesame seeds and sliced green onions before serving.