In a bowl, combine the thinly sliced beef with soy sauce, sesame oil, minced garlic, ground cumin, ground coriander, salt, and pepper. Mix well until the beef is evenly coated, then cover the bowl and let it marinate in the refrigerator for at least 30 minutes.

In a large pot, bring 4 cups of chicken stock to a gentle simmer over medium heat. Whisk in 2 tablespoons of peanut butter until the mixture is smooth and creamy. Allow the broth to heat through for about 10 minutes, stirring occasionally.

While the broth simmers, prepare your rice noodles according to the package instructions. Typically, this involves soaking them in hot water for about 5-10 minutes until tender. Once done, drain the noodles and set them aside.

Heat a large, non-stick frying pan over high heat. Add the marinated beef in batches and stir-fry for about 2-3 minutes until browned and just cooked through.

Once the beef is cooked, carefully add the drained rice noodles to the simmering broth. Gently mix the sautéed beef into the broth, ensuring everything is well combined.

Ladle the soup into bowls, ensuring each portion has a good balance of noodles, beef, and broth. Top with fresh bean sprouts, cilantro, lime juice, and diced red chili, if using. Serve immediately.