This is actually called banh mi bo kho or bo kho ca rot in vietnamese and it is flavorful I made it with coconut water to lighten the recipe, but you can use beef stock for a meatier flavor. If you are lucky enough to have leftovers, you'll find that the flavors really kick in and it tastes even better.
Combine the fish sauce, five-spice powder, and sugar in a dish. Add the short ribs or oxtail, coating well, and allow to marinate, refrigerated, for 4 to 6 hours.
Heat together the oil, onions, garlic, and ginger in a sand pot or heavy-bottomed pot over medium-low heat.
Cook the ingredients, stirring often, until fragrant and golden, about 10 minutes. Add the marinated meat, scraping the marinade from the dish into the pot. Add the coconut juice or beef stock and cover the pot. Bring to a boil over medium heat. Add the lemongrass, star anise, cinnamon stick, chilies, and annatto seeds and simmer covered (occasionally turning the meat) over low heat for 2 hours.
Add carrots and continue braising until the meat falls off the bones, 1-1/2 to 2 hours more.
Skim off the fat and serve with a french roll on the side.