In a small bowl, whisk together ¼ cup soy sauce, broth, brown sugar, rice wine vinegar, and 2 teaspoons cornstarch until combined. Set aside.
In a medium bowl, add chicken, sesame oil, Chinese 5 spice, pepper, and remaining 1 teaspoon corn starch; toss to coat until cornstarch dissolves. Let marinate 15 minutes.
Heat vegetable oil in a large skillet or wok over medium-high heat.
Add chicken to pan; cook 2 minutes. Add garlic and ginger to pan; toss to combine with chicken.
Cook until chicken is browned on all sides, 2 to 3 minutes more. Add mushrooms, haricot vert, and bell pepper, stirring to combine.
Cover, and cook until vegetables are tender, about 5 minutes. Uncover; whisk sauce, and pour into skillet.
Cook, stirring often, until sauce thickens, about 2 minutes.
Remove from heat; stir in cooked noodles until combined and coated with sauce.
Notes
If you can’t find lo mein noodles at your grocery store, then by all means use linguine or fettucini noodles instead.
Since you’re cooking the noodles ahead of time, run them under hot water (be sure to drain!) before adding them to the sauce so they don’t stick together.
I used yellow bell pepper because I like the color, but feel free to use red or green instead.
Haricot vert are a slim and tender variety of green bean; you can easily substitute any stringless green bean you can find.