Marinate beef slices and let sit for 20 minutes. Cook sweet potato noodles according to package instructions, then drain and set aside.
Blanch spinach for 1 minute, drain, rinse under cold water, squeeze out excess water, and set aside.
Sauté onion and shiitake until translucent and cooked through, then set aside. Cook carrot and red bell pepper until tender-crisp, then set aside.
Stir-fry marinated beef until cooked through and slightly caramelized, then set aside.
Mix all the ingredients for sauce in a small bowl.
In a large mixing bowl, combine cooked sweet potato noodles, sauce, and cooked ingredients.
Toss until well combined.
Garnish with toasted sesame seeds. Enjoy!