Step 1: Marinate the Beef. In a medium bowl, combine the thinly sliced beef with 1 tablespoon soy sauce, 1 tablespoon cornstarch, and the baking soda (if using). Mix well to coat and let marinate for at least 15 minutes at room temperature, or up to 30 minutes in the refrigerator.
Step 2: Prepare the Vegetables. Chop all vegetables into uniform, bite-sized pieces. Mince the garlic and ginger.
Step 3: Whisk the Sauce. In a small bowl, whisk together the remaining 1 tablespoon soy sauce, beef broth, oyster sauce (if using), rice vinegar, brown sugar, 1/2 teaspoon sesame oil, and 1 tablespoon cornstarch until smooth. Set aside.
Step 4: Sear the Beef. Heat 1 tablespoon of vegetable oil in a large wok or heavy-bottomed skillet over high heat until shimmering. Add half of the marinated beef in a single layer (do not overcrowd the pan). Cook for 1-2 minutes per side until nicely browned. Remove the beef and set aside. Add remaining 1 tablespoon of oil and repeat with the remaining beef. Remove and set aside.
Step 5: Stir-Fry Aromatics and Harder Vegetables. Add the minced garlic and ginger to the wok. Stir-fry for 30 seconds until fragrant. Add the broccoli florets and sliced carrots. Stir-fry for 3-4 minutes until they begin to soften but are still crisp-tender.
Step 6: Add Softer Vegetables. Add the sliced red bell pepper and snap peas to the wok. Continue to stir-fry for another 2-3 minutes until they are bright in color and crisp-tender.
Step 7: Combine and Sauce. Return the seared beef to the wok with the vegetables. Give the prepared sauce a quick whisk again (as cornstarch can settle) and pour it over the beef and vegetables. Stir continuously for 1-2 minutes until the sauce thickens and coats everything beautifully.
Step 8: Serve. Remove the beef stir fry with vegetables from the heat. Serve immediately over steamed rice or noodles. Garnish with sesame seeds and sliced green onions, if desired.