In a medium bowl, whisk together the cornstarch, water, and soy sauce or tamari. Place the cleaned shrimp in the bowl and set aside.
Heat a wok or large, heavy skillet on high. Add the cooking oil to the hot pan. Add the peppers, onion slices, garlic, and ginger to the oil. Stir fry until aromatic and the onions begin to wilt, approximately 2 to 3 minutes.
Stir in the shrimp and cook until just barely starting to turn pink, about 30 seconds to 1 minute.
Add the sha cha sauce and stock to the wok or pan. Simmer until the liquid has thickened and the shrimp are cooked, but not rubbery.
Serve immediately over white or brown rice. Top with scallions and sesame seeds.