To make the sauce base, sprinkle the cornstarch over 1/3 cup water in a small bowl and whisk to dissolve the cornstarch. Add the broth, soy sauce, vinegar, sugar, and red pepper flakes and mix well. Set aside.
Heat a 12 inch nonstick skillet over high heat. Add 1 tablesppon of the oil and swirl the skillet to slick the bottom with oil. Add the shrimp and stir-fry just until the shrimp turn opaque, about 2 minutes. Using a slotted spoon, transfer to a plate.
Add the remaining 1 tablespoon ili to the skillet and heat over medium high heat. Add the red pepper and stir until it begins to soften, about ` minute, Add the scallions and stir until they begin to wilt, about 1 minute. Add the pineapple and cook, stirring occasionally, until heated through, about 2 minutes. Stir in the shrimp and garlic and stir-fry until the garlic gives off its aroma, about 1 minute. Pour in the sauce base and brin to a boil to thicken the sauce stirring occasionally.
Serve hot, spooned over rice.