In a mixing bowl, whisk together the soy sauce, honey, brown sugar, minced garlic, grated ginger, sesame oil, and red pepper flakes (if using) until well combined.
Add the steak cubes to the marinade, ensuring they are evenly coated. Cover and let it marinate for at least 30 minutes at room temperature or up to 2 hours in the refrigerator for deeper flavor.
After marination, remove the steak from the marinade, reserving the liquid. Pat the steak cubes dry with a paper towel to ensure they sear well.
Heat a large skillet over medium-high heat. Add a drizzle of oil (if needed) and, when hot, add the steak bites in a single layer without overcrowding. Sear them for about 2-3 minutes on each side until they develop a nice caramelized crust. Season with salt and pepper.
Once the steak is cooked to your liking, lower the heat to medium and add the reserved marinade to the pan, mixing in the cornstarch to thicken the sauce. Stir continuously for 2-3 minutes until the sauce becomes shiny and coats the steak bites.
Remove from heat and transfer the steak bites to a serving dish.
Garnish with sliced green onions and a sprinkle of sesame seeds for an added crunch and flavor.