Cut off the ends of the fresh lemongrass first, then tenderize it by using a rolling pin to roll over the lemongrass, similar to rolling out dough, or you can also use a meat tenderizer.
Place 2 tablespoons of vegetable oil in a pot, add ¼ of the sliced onion, remaining chopped shallot, tenderized lemongrass and let it cook for about 3 minutes.
Add the marinated cubed beef to the pot to sear the beef. After the beef is seared, add the can of coconut soda or coconut water.
Cover the pot and cook until the meat is tender (approximately 1 hour and 45 minutes).
When the beef is cooked, add in the cut up carrots. If some of the liquid has evaporated, you can add in 2 cups of chicken broth and continue cooking with the lid on until carrots are cooked through. Add more chicken broth if you want the bo kho sauce to be thinner (i.e. if you plan to serve it with noodles). Add less chicken broth if you want the bo kho sauce to be thicker (i.e. if you plan to serve it with a side of baguette).
When the carrots are cooked through, (approximately 30 minutes) add the fish sauce to taste.
Add the rock sugar. Mix well and remove the stalks of lemongrass before serving.