Put the meat in a dish.
In a container mix 2 tablespoons of fish sauce and a tablespoon of lemon juice. Pour everything over the meat, turn it over once or twice, then leave it in the refrigerator for at least 3 hours.
Drain the meat and cook on a grill to your liking, rare or medium but not overcooked. Let it rest for 5 minutes before cutting it into thin slices.
Mix the cucumber, chilli, onion, tomatoes, mint and coriander in a salad bowl. Whisk together the remaining lemon juice and fish sauce, palm sugar, soy sauce and garlic to obtain a homogeneous sauce.
Pour it over the salad and toss gently. Divide the salad on the plates and arrange the slices of rump steak on top.
Serve immediately.
