Prepare the Sauce: In a bowl whisk together soy sauce, cold water, cornstarch, ginger, garlic, and brown sugar until completely combined. Set aside.
Heat 2 tablespoons of canola oil in a large skillet over high heat. Add the asparagus and stir-fry for about 2 minutes (or until crisp-tender). Add the red bell pepper and onion. Stir-fry for about 2 more minutes, or until crisp-tender. Remove to a plate and set aside.
Add meat to the skillet in a single layer (still over high heat) and cook for about 1 minute – do not stir or move the meat while it is browning. Flip the meat, cook on the other side for about 30 more seconds, and then remove to the plate with the vegetables. Drain the fat from the skillet.
Pour the sauce into the skillet and bring to a boil; reduce heat to low and simmer while whisking until the sauce thickens (about 1-2 minutes). If the sauce gets too thick, just add additional water until it reaches desired consistency. Return beef and vegetables to the skillet and stir to coat with the sauce. Serve with rice and garnish with sliced green onions, if desired.