In a medium bowl, combine the thinly sliced beef with 1 tbsp soy sauce, 1 tsp sesame oil, 1/2 tsp black pepper, and 1 tsp cornstarch. Toss to coat evenly and set aside to marinate for at least 10 minutes while you prepare other ingredients.

Heat 1 tbsp vegetable oil in a large pot or Dutch oven over medium-high heat. Add the minced garlic and grated ginger, and sauté for 1 minute until fragrant.

Add the marinated beef to the pot in a single layer (you might need to do this in batches if your pot is not large enough to avoid overcrowding). Cook for 1-2 minutes per side until lightly browned. Remove the beef from the pot and set aside.

Pour the beef broth into the same pot, scraping up any browned bits from the bottom. Add 2 tbsp soy sauce, rice vinegar, sugar, white pepper, and rehydrated and sliced shiitake mushrooms (if using). Bring the broth to a rolling boil.

Add the egg noodles (or ramen noodles) and julienned carrots to the boiling broth. Cook according to package directions, typically 3-5 minutes for fresh noodles, or slightly longer for dried.

Stir in the baby spinach until wilted, about 30 seconds. Return the cooked beef slices to the pot and gently stir to combine and heat through for about 1 minute.

Ladle the hot soup into individual bowls. Garnish generously with thinly sliced green onions and fresh cilantro. Drizzle with chili oil if desired. Serve immediately.