In a small bowl or wide-mouth jar, whisk cornstarch and soy sauce until smooth. Add chicken broth, brown sugar, vinegar, hoisin, oyster sauce, Sriracha (if using), white pepper, and 1 tbsp sesame oil. Whisk until smooth and set aside.
Thinly slice onion and bell pepper. Finely chop the broccoli. Measure out the carrots. Slice the green onions. Mince ginger and garlic if not using paste.
Heat 1 tbsp sesame oil in a large wok or deep pan over medium-high heat. Add the onion and sauté for 4 minutes. Push onions to the edges. Increase heat to high, add another tbsp oil, then add ground beef to the center. Let sit for 30 seconds, then break it up with a wooden spoon. Cook until no longer pink, about 5–7 minutes. It’s okay if it’s not fully cooked yet.
While beef cooks, boil a large pot of water (do not add salt). Discard seasoning packets from ramen. Cook noodles for 1 minute and 30 seconds less than package instructions. Drain, toss with 1 tbsp sesame oil, and set aside.
Reduce pan heat to medium-high. Add bell pepper, broccoli, carrots, garlic, and ginger. Stir-fry for 3–4 minutes until veggies are crisp-tender.
Stir the sauce again and pour it into the pan along with cooked noodles. Use tongs to toss everything together for 2–4 minutes, or until the sauce is absorbed and thickened slightly. Veggies should still have a slight crunch.
Taste and adjust with more soy sauce, Sriracha, or pepper if needed. Top with green onions and optional garnishes. Serve hot.
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Notes
Note 1: Garnish ideas — sliced green onions, a drizzle of sesame oil, and/or toasted sesame seeds.Storage: Store leftovers in an airtight container in the fridge for up to 2 days. Reheat in a skillet or microwave with a splash of water to loosen the sauce.