Toss the beef with fish sauce and refrigerate overnight.
Preheat the oven to 200°C.
Drain excess fish sauce from the beef.
Combine beef, coconut milk, galangal, lemongrass, ginger, and lime leaves in a roasting tray.
Cover the tray with foil and bake for 40 minutes.
Increase oven temperature to 220°C.
Remove foil, turn the beef, and cook for another hour until liquid evaporates and meat browns.
Transfer meat to a plate to cool at room temperature.
Slice the cooled beef into thick pieces.
Pound chillies, lime juice, fish sauce, and sugar in a mortar to make the dressing.
Toss shredded mango, beansprouts, and shallots together in a large bowl.
Mix in the dressing and beef then rest for 5 minutes.
Toss in the herbs and serve topped with crushed roasted rice.