Put the steak in the freezer for 15 minutes while you prep your vegetables. This will make it easier to slice.
Seed, derib and cut the bell pepper into 1/4 inch strips. Trim the asparagus spears and cut on the diagonal into 1 1/2 inch lengths.
Fill a medium sized saucepan halfway with water and bring to a boil. Blanch the cut asparagus for 2 minutes. Drain, rinse with cold water to stop the cooking, and set aside.
Whisk all of the sauce ingredients together in a small bowl and set aside.
Remove the flank steak from the freezer and slice in half lengthwise (along the grain) and then cut into thin strips (across the grain), 2 inches long and 1/4 inch wide.
Heat 1 tablespoon of oil in a wok (or a thick-bottomed pot that can handle high heat) on high heat until shimmery. Add the asparagus and fry, stirring constantly, for 2 minutes or until lightly browned. Remove the asparagus from the pan and set aside.
Heat another tablespoon of oil in the pan on high heat until shimmery. Stir fry the beef strips in batches, for 2 to 3 minutes per batch, until they are browned but still a little pink inside. Return the first batches of beef strips to the pan along with the bell pepper strips. Stir and toss over high heat until the bell pepper strips are just beginning to wilt, 1-2 minutes.
Quickly stir the cornstarch liquid and add it to the pan along with the stir fry sauce. Cook until sauce thickens, 1-2 minutes. Return the asparagus to the pan, toss to evenly coat and serve over steamed rice.