Start with the sauce: In a bowl or a measuring cup with a spout, whisk together the soy sauce, water, and brown sugar.
Continue with the sauce: In a small saucepan, warm the canola oil over medium heat. Add the garlic and ginger and cook until fragrant, about 1 to 2 minutes. Carefully pour in the soy sauce mixture, then increase the heat to bring it to a boil. Reduce heat to low. Simmer gently, stirring occasionally, for 5 minutes. Remove from the heat and set aside to cool while you prep the remaining ingredients.
Move on to the beef and broccoli: Thinly slice the sirloin across the grain into super thin strips (1/4-inch thick or thinner) that are about 1 1/2 inches in length and place in a medium bowl. Toss with 1 teaspoon sesame oil and 1 teaspoon of the cornstarch. Let marinade for 10 minutes or refrigerate for up to 1 hour.
Chop the broccoli into small florets and thinly sliced the green onions.
Once the beef is marinated, heat 1 tablespoon canola oil over medium-high heat. Once the oil is hot, with tongs, carefully lower the beef into the hot oil, shaking off any excess marinade (hang onto the bowl). Stir fry the beef until it is cooked through, about 2 minutes. Watch out for splatters and use a splatter screen if you have one.
While the beef cooks, stir the remaining 1 teaspoon cornstarch into the bowl with the sauce you used to marinade the beef. Keep near the stove.
Remove the cooked beef to a clean plate or bowl. Add the remaining 1 tablespoon canola oil to the skillet, then add the broccoli. Stir to coat the broccoli with the oil, then carefully pour in the water. Stir fry until crisp tender, about 4 minutes.
Return the beef strips and any juices that have collected on the plate to the pan. Pour the reserved sauce over the top. Reduce the heat to low, and then cook for 1 minute, stirring constantly. Stir in the salt, green onions, and red pepper flakes. Remove from the heat and taste for seasoning, adding more salt as desired.
Sprinkle with sesame seeds. Serve warm over rice with additional red pepper flakes to taste.
Notes
For an extra-spicy stir fry, add 1 sliced red Thai pepper to your sauce or to the beef when frying.
TO STORE: Refrigerate leftovers in an airtight storage container for up to 3 days.
TO REHEAT: Gently rewarm beef and broccoli in a skillet on the stovetop over medium-low heat or in the microwave.
TO FREEZE: Freeze leftovers in an airtight freezer-safe storage container for up to 3 months. Let thaw overnight in the refrigerator before reheating.