Blanch the udon in boiling water for 1-2 minutes to loosen them up. Drain, rinse with cold water, and toss with a drizzle of sesame oil so they don't stick. Set aside.
Heat a large pan over medium-high heat and add a drizzle of oil. Add the beef season with a pinch of salt, and brown it.
When it's almost cooked through, pour in about 1 ½ tablespoons of the stir-fry sauce, toss to coat, then remove the beef from the pan and set aside.
If the pan is looking a little dry, add a touch more oil. Add the onions and carrots and cook for 2-3 minutes, then season with a pinch of salt. Add the garlic and bok choy and sauté for about a minute. Add the udon noodles, beef, stir-fry sauce, and green onions, and mix everything together until well combined.
Top with chives and a drizzle of chili oil.