In a bowl, toss thinly-sliced beef with 1/4 teaspoon salt, 1/8 teaspoon pepper, 1 teaspoon minced garlic and 1 teaspoon oil. Set aside for about 10 minutes while preparing other ingredients.
Combine oyster sauce, soy sauce, ketchup, sugar and 1/8 teaspoon pepper in a bowl to make the stir-fry sauce. Taste and adjust the level of sweetness to your liking
Cook pasta in boiling salted water according to package instructions, but undercook by 1 minute. Rinse under cold water, drain and toss with some olive oil to prevent sticking.
Heat a pan over medium heat, add some oil. Once the oil is hot, add half of the remaining minced garlic, stir and cook for a few seconds until fragrant. Then add sliced onions, stir briefly and then add julienned carrots and bell pepper. Stir and cook for 1-2 minutes, then add baby bok choy. Continue to stir and cook for another minute until all veggies are crisp-tender. Transfer to a plate.
To the same pan, add a bit more oil. Sauté the remaining minced garlic for a few seconds until fragrant. Add beef to the pan in a thin layer (increase the heat to medium-high if needed), sear, stir and cook until beef is 70-80% cooked. Add pasta and half of the stir-fry sauce. Toss and cook for a few seconds to coat beef and pasta with sauce.
Add the vegetables to the pan and give everything a quick toss. Then add the remaining sauce and continue to stir-fry quickly until everything is cooked through. Transfer to serving plates and serve immediately.