Heat 1 tablespoon olive oil in a large pot over medium-high heat. Add the sliced beef and season lightly with salt and pepper. Cook for 2–3 minutes, stirring occasionally, until the meat browns slightly but remains tender. Transfer the beef to a plate and set aside.

In the same pot, add the remaining 1 tablespoon olive oil. Add the white and light green parts of the scallions, garlic, and ginger. Cook for 1–2 minutes, stirring frequently, until fragrant.

Stir in the mushrooms, soy sauce, and chili sauce. Cook for another 2 minutes until the mushrooms start to soften and absorb the flavors.

Pour in the vegetable or beef broth, water, and add the cinnamon stick. Bring the mixture to a gentle boil, then reduce the heat and simmer for 5–6 minutes to infuse the spices.

Remove and discard the cinnamon stick. Return the cooked beef to the pot, along with the lime juice and sesame oil. Add the cooked vermicelli noodles and stir gently to combine. Simmer for 1–2 minutes just to warm everything through.

Ladle the soup into bowls and top with the reserved scallion greens. Add a few drops of chili oil or a sprinkle of red pepper flakes if you enjoy extra heat.