Heat the olive oil over medium-high heat in a large pot. Then add the onion and garlic and sauté a few minutes until the onions are soft and translucent.
Add the remainder of the broth ingredients. Bring to a boil, then reduce the heat to medium-low and let simmer for 45 minutes.
Strain out all the broth ingredients, then return the broth to high heat and bring up to a boil.
Add the vermicelli rice noodles and thinly sliced onion, and cook just until the rice noodles are tender (1–2 minutes).
Add the thinly sliced beef and season the broth to taste with freshly cracked black pepper.
Serve with the scallions, bean sprouts, lime wedges, and Thai basil on the side for each person to garnish their bowls individually.
