This amount of sauce is good for serving over rice. If serving without rice, use only 1/2 - 3/4 of the sauce when adding to the pan.
In a small bowl, mix sauce all ingredients except cornstarch, and set aside. Rinse shrimp under cold water and blot dry. (If shrimp are jumbo or larger, cut into 1" pieces.) Place in a medium-sized bowl and spoon a couple tablespoons of sauce over the shrimp and stir to mix well. Let sit about 10 minutes. Whisk the cornstarch into the remaining sauce, mixing until no lumps remain.2.
Heat 2 tablespoons oil in a large skillet over high heat until very hot. Add the julienned ginger and stir about 30 seconds. Add the carrots and broccoli, cooking another 30 seconds. Add the red onion and cook, stirring, for 2 minutes.3.
Push the vegetables to the side of the pan and add 1 tablespoon oil to the center. Once oil is hot, about 30 seconds, add the shrimp and scallions and cook 1 minute, stirring frequently.4.
Re-stir the sauce in its bowl to re-blend the cornstarch and pour into the skillet. Let it reach a simmer, then stir everything together for about 1 minute, until sauce is thickened.*See note, below.5.
Stir in bell pepper or corn, cooking 1 minute more.6.
Serve while hot, over cooked brown or white rice.