Whisk together all ingredients for the sauce and set aside.
Toss shrimp pieces with cornstarch and salt to evenly coat. Heat oil in a large skillet or wok over medium-high heat until shimmering hot.
Add shrimp in a single layer and cook, undisturbed, until golden on the bottom. Flip and cook another minute (don’t worry about cooking it through yet). Transfer shrimp to a plate.
Add green beans and green onions to same skillet and stir fry on high heat until blistering, about 4 minutes. Add garlic and cook a minute more.
Add shrimp back to the skillet along with the sauce mixture, stirring to coat. Let it come to a simmer and cook until thickened, 2-3 minutes more.
Taste and add red pepper flakes if you like it spicier