For Sauce, combine 1 1/4 cups broth, cornstarch, soy sauce, ginger, garlic and red pepper flakes in a pint-size jar with lid; shake well and set aside.
For Stir Fry, heat a dry 12-inch skillet over high heat 1 minute (medium-high if using non-stick). Add sliced meat in an even layer. Cook, without stirring, 2 minutes. Stir once, then continue cooking until liquid evaporates and meat starts to brown on edges and stick to pan, about 3 to 5 minutes, stirring only one or two more times. If pan becomes dry, add broth 1 tablespoon at a time. Meat may still be slightly pink in center.
Add broccoli, onion and 2 tablespoons broth. Stir until vegetables are hot, about 3 minutes, adding additional broth 1 tablespoon at a time, as needed, if pan becomes dry.
Shake sauce mixture again, checking that cornstarch is fully incorporated, and pour into skillet. Simmer, 2 to 3 minutes, stirring frequently, until sauce is thickened and vegetables are desired doneness. Serve over hot rice or noodles with toppings, as desired.