Start by making the sauce. In a small bowl, whisk soy sauce, water, and brown sugar.
In a small sauce pan, heat oil over medium heat. Add garlic and ginger, cook for 1-2 minutes until fragrant. Pour in soy sauce mixture and bring to a boil.
Reduce heat to low and simmer for 5 minutes, stirring occasionally.
Remove from heat and let cool.
Thinly slice the sirloin into ¼ inch thick strips, add to a medium sized bowl.
Toss beef with 1 tsp sesame oil and 1 tsp cornstarch. Let sit for 10 minutes.
Meanwhile, chop the broccoli into small florets and slice the green onions thinly. Now is also a good time to make quick-cook rice (such as minute rice).
In a large skillet, heat 1 tbsp oil over medium-high heat. Add beef to skillet (careful! Oil may pop.). Cook about 2 minutes. Move beef to a plate or bowl and set aside.
Stir remaining 1 tsp cornstarch into sauce.
Add 1 tbsp oil to skillet along with the broccoli. Stir to coat, then pour in water. Cook until broccoli is tender, about 4 minutes.
Add beef strips to skillet. Pour sauce over the top.
Reduce heat to low and cook for 1 minute, stirring constantly.
Stir in salt, green onion, and red pepper flakes. Remove from heat.
Serve over rice and top with sesame seeds.
