Place the beef into a bowl and toss with the fish sauce. Cover and place in the fridge overnight.
The next day, when ready to cook, preheat the oven to 200C/400F/Gas 6.
Remove the beef from the fridge and drain off any excess fish sauce.
Transfer the beef into a roasting tray and add the coconut milk, galangal, lemongrass, ginger and lime leaves. Stir together and cover with foil.
Place in the oven and cook for 40 minutes.
After 40 minutes turn the oven up to 220C/430F/Gas 7. Remove the foil and turn the beef over before cooking for a further hour. Check from time to time that there is enough liquid, adding a little water if it's too dry, and turning the beef occasionally.
After an hour the liquid should have evaporated and the meat browned. Remove from the oven and transfer the meat onto a plate to cool - do not refrigerate unless using the next day as it is better served at room temperature.
When cool, slice the beef thickly.
To make the dressing, place the chillies, lime juice, fish sauce and palm sugar into a pestle and mortar and pound to combine.
Peel and shred the mango or papaya and put in a large bowl with the beansprouts and sliced shallots.
Add a generous amount of the dressing and the sliced beef to the mango or papaya and stir to combine. Leave to one side for five minutes to allow the flavours to come together.
When ready to serve, add the herbs and toss to combine. Transfer to a serving plate and top with the crushed roasted rice.