In a bowl, whisk together the soy sauce, brown sugar, mirin and garlic. Set aside.
Heat a cast iron pan over medium-high heat.
While the pan is heating, season the sliced steak with ½ tsp salt.
Add the avocado oil and when it begins to shimmer add the sliced steak. Sear on each side for about a minute so it gets a golden color.
Reduce the flame to medium, add the soy sauce mixture, broccoli and butter, and cook, stirring for 2 minutes.
in the same bowl that you made the soy sauce mixture, whisk together the cornstarch and water.
Increase the flame to high and pour in the cornstarch mixture. Stir until the sauce thickens and fully coats the steak and broccoli.
Serve over steamed white rice.