1. Put steak in freezer for 15 minutes while prepping vegetables. This will make it easier to slice. Fill medium-sized saucepan halfway with water and bring to a boil. Blanch cut asparagus for 2 minutes. Drain, rinse with cold water to stop cooking, and set aside.
2. For sauce. Whisk sauce ingredients together in small bowl and set aside.
3. Heat 1 Tbsp oil in wok (or a thick-bottomed pot that can handle high heat) on high heat until shimmery. Add asparagus and fry, stirring constantly, for 2 minutes or until lightly browned. Remove asparagus from pan and set aside.
4. In same pan, stir fry beef strips in batches, for 2 to 3 minutes per batch, until browned but still a little pink inside. Remove to plate. Add bell peper to pan, stir and toss over high heat until bell pepper strips are just beginning to wilt, 1-2 minutes. Add steak pieces and asparagus to pan, quickly stir cornstarch liquid and add it to pan along with stir fry sauce. Cook until sauce thickens, 1-2 minutes. Toss to evenly coat.
Serve warm over steamed rice or noodles, garnished with scallions.