Place the cubed sirloin in a bowl and sprinkle the baking soda over it, tossing until every piece is lightly coated. Let it sit for 20 to 25 minutes at room temperature.
Transfer the beef to a fine-mesh strainer and rinse thoroughly under cold running water to wash off all the baking soda. Pat the cubes completely dry with paper towels.
Return the rinsed, dried beef to a clean bowl. Add the soy sauce, cornstarch, and sesame oil, then toss until each cube is evenly coated. Set aside for at least 10 minutes.
In a separate bowl, whisk together the soy sauce, oyster sauce, Shaoxing wine, brown sugar, beef broth, cornstarch, and coarse black pepper until the cornstarch fully dissolves and no lumps remain.
Heat 1 tablespoon of avocado oil in your wok over high heat until it shimmers. Add the onion and tougher broccolini stems and stir-fry for about 1 minute.
Add the mushrooms and red bell pepper and stir-fry for another 2 to 3 minutes, until the vegetables are bright and crisp-tender. Transfer to a clean bowl and set aside.
Wipe the pan clean, return it to high heat, and add the remaining 2 tablespoons of avocado oil. Lay the beef in a single, uncrowded layer and do not touch it for 3 minutes.
Flip the pieces and sear another 1 to 2 minutes until browned on the outside. Sear in two batches if your pan is small. The total sear should be around 4 to 5 minutes.
Add the minced garlic to the beef and stir for about 30 seconds, just until fragrant. Give the sauce a quick re-whisk and pour it into the pan.
Let the sauce bubble and thicken for about 1 minute. Return the vegetables to the pan and toss everything together until glossy and coated, about 1 minute more.
Taste and add another crack of black pepper if you want it sharper, then serve immediately over hot rice.