Marinate the beef in a medium bowl with the soy sauce, sesame oil, baking soda, salt, and freshly cracked black pepper. Let it sit for 5-10 minutes.

In a separate bowl, mix the soy sauce, brown sugar, sesame oil, and freshly ground black pepper. You can microwave it for 20 seconds to help dissolve the sugar. Set aside.

Bring a large pot of water to a boil and cook the sweet potato noodles for 5-6 minutes until softened and chewy. Drain, rinse under cold water to remove excess starch, then transfer to a large mixing bowl.

Heat a pan over medium-high and cook the beef for 1-2 minutes, then set aside.

In the same pan, add the garlic, onion, and shiitake mushrooms. Stir-fry for 1-2 minutes, season with a pinch of salt, then remove and set aside.

Still in the same pan, add the carrot and red bell pepper and stir-fry for 2-3 minutes until softened. Add a pinch of salt, then remove and set aside.

In a medium pot, bring water to a boil and add the spinach. Boil for 1 minute until just cooked, then drain and squeeze out the excess water.

To the large bowl with the noodles, add the cooked onion, mushrooms, carrot, red bell pepper, spinach, and beef. Pour in the sauce, sprinkle in the sesame seeds, and quickly toss until everything is well coated. Use kitchen scissors to roughly cut the noodles a few times to trim them down. Serve immediately.