Mix the sauce: In a small bowl, whisk soy sauce, rice vinegar, brown sugar, sesame oil, red pepper flakes (if using), and broth.
Make the slurry: In another small bowl, stir cornstarch with cold water until smooth. Keep both bowls by the stove.
Pat greens dry: Spinach should be very dry so the stir-fry sears, not steams.
Sear mushrooms (2–3 min): Heat oil in a large skillet or wok over medium-high until shimmering. Add mushrooms in an even layer; cook undisturbed 90 seconds, then stir and cook 1–2 minutes more until golden at the edges.
Aromatics (30–45 sec): Add garlic, ginger, and scallions; stir constantly until fragrant. ️
Sauce & thicken (45–60 sec): Pour in the sauce; when bubbling, stir in the slurry. Cook, stirring, until glossy and slightly thick.
Wilt spinach (60–90 sec): Add spinach in 2–3 handfuls, tossing until just wilted and bright.
Taste & serve: Adjust salt/acid (a dash more soy or vinegar). Plate over rice or noodles. Sprinkle sesame seeds and scallion greens; add a squeeze of lime if you like.