Toss the thinly sliced beef with sliced onions, sesame seeds, sesame oil, and soy sauce. Let it marinate for a bit.
Prepare the vegetables by shredding them thinly (lettuce, carrot, cucumber, cabbage, green mango).
Cook the marinated beef in butter on high heat in small batches until golden and juicy, about 3-4 minutes max.
Assemble the bowl with vermicelli noodles, the prepared vegetables, the cooked beef, herbs (mint, Vietnamese mint, coriander), fried shallots, and peanuts (if using).
Drizzle the nước chấm (Vietnamese dipping sauce) over the assembled bowl.
