Cut the steak into small cubes no more than 1 inch in size. Try to do this as evenly as possible so the bites cook evenly. Set the steak bites in a medium bowl.
In a small bowl, whisk together the coconut aminos, sweetener, garlic, ginger, and red pepper flakes. Pour over the steak bites and toss to coat. Let marinate 30 minutes at room temperature or 2 hours in the fridge.
Heat a large skillet over medium high heat. Once hot, add 1 tablespoon of the sesame oil. Using a slotted spoon, add half of the steak bites in a single layer. Let them cook undisturbed for about 2 minutes, then quickly flip them over and cook on the second side for another minute or so. They should be seared on the outside but still a little pink and tender in the center.
Transfer to a bowl and repeat with the remaining oil and the remaining steak bites.
Once all the steak is cooked and removed from the pan, pour the reserved marinade into the pan and bring to a boil. Cook until reduced and thickened, 4 to 5 minutes. Pour over the steak bites or serve on the side.
Garnish with toasted sesame seeds and cilantro.
If you use coconut aminos, you may want to add a little salt to the finished recipe.