In a bowl, combine all Sauce ingredients. Use hand whisk to be sure cornstarch is fully dissolved into the sauce mixture (any bit of undissolved cornstarch will cause gummy lumps in dish.) Set sauce aside.
In a large, deep pan, add 3 TB olive oil over medium-high heat until oil is hot. Add garlic and onion, stirring 2 minutes until fragrant and translucent.
Add broccoli florets, cooking about 2 minutes or just until just tender. Transfer vegetable mixture into a dish and set aside.
Add 2 TB olive oil into the now-empty pan, over medium high heat. Meanwhile, pat-dry all pieces of sliced sirloin steak to remove any excess moisture. Once oil is hot, add beef in a single layer in to the pan.
Let beef brown, uninterrupted, for 3 minutes for bottom to brown. Once bottom of beef slices are browned, give it a good stir.
Add back the cooked broccoli/garlic mixture and the sauce you prepared. Stir to incorporate well and continue to cook for another minute. Taste and add red pepper flakes, if desired.
Notes
Tamari soy sauce, mirin, seasoned rice vinegar, and oyster sauce are usually found in the Asian aisles of most major grocery stores these days.
Slice the beef thinly across the grain. This keeps the meat tender and helps it cook quickly without becoming tough.
Pat the beef dry before searing. Removing excess moisture ensures a nice brown crust instead of steaming the meat.
Use high heat for stir-frying. A hot pan helps the beef sear fast and keeps the broccoli crisp and bright.
Don’t overcrowd the pan. Cook the beef in a single layer so it browns evenly and develops good flavor.
Dissolve the cornstarch completely in the sauce before cooking. This prevents lumps and gives you a smooth, glossy finish.
If you prefer a bit of char for the broccoli, let florets brown and flip only occasionally, without stirring too much.
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