Make the sauce by combining the soy sauce, vinegar, and sweetener in a small bowl. Set aside.

Melt the butter in a cast iron pan over a medium-high heat. Add the garlic and sauté for a few minutes, stirring frequently. Next, add in the steak and turn to coat them in the garlic butter. 

Pour the sauce into the pan and cook until the steak is cooked to your liking. 

Garnish with finely chopped green onions and season with salt and/or black pepper, if liked.