Trim the oxtail of any excess fat and season with salt.
In a large Dutch oven or other heavy-bottomed pot set over high or medium-high heat, sear the oxtail until all the sides are browned, approximately 2 minutes per side. Do this in small batches to avoid overcrowding the pot (crowding the pot will result in the oxtail steaming rather than searing).
Once all the oxtail is seared, remove the oxtail and add 1 cup of water to the pot. Use a spatula to scrape and stir the bits of caramelized meat juices stuck to the bottom of the pot.
Return the oxtail to the pot. Add the leek and an additional 3 quarts (12 cups)of water.
Bring the contents to a boil for 15 minutes, skimming off any foam that develops. (I use this super fine mesh strainer to remove any foam or scum at the top.)
Add the fish sauce, lower the heat, partially cover the pot, and simmer the stock for 4 hours. Check on the stock every so often to skim off any foam that develops on the surface.
While the stock cooks, prepare the vegetables as follows:Chop the onion into ½-inch pieces.Peel the kohlrabi and cut into ¾-inch pieces.Peel and slice the carrots into ¾ inch pieces.Cut cabbage into 1-inch pieces.Quarter lengthwise baby bok choy. Using a peeler, remove the fibrous strings from the celery. Slice the celery into ½- to 1-inch pieces. After you prepare them, place them in the fridge until you’re ready to assemble your soup.
After the oxtail has simmered on the stovetop for four hours, the meat should be fork tender. Fill a large bowl with water and ice and transfer the oxtail into the ice bath and set aside. (The ice bath is optional—Mama Pham does it to keep the oxtail from discoloring.)
Strain the stock into another large pot or bowl. If the stock ends up being less than 2 quarts (or 8 cups), add enough water to make up the difference.
Rinse out the pot, then return the strained stock back into the pot. Return the oxtail to the pot. (If you prefer your oxtail boneless, you may remove the meat from the bones, returning only the meat to the stock.)
Bring the stock and oxtail to a boil, then add the onions, kohlrabi, carrots, and celery. Cook for approximately 15 minutes, skimming off any foam that develops.
Add the cabbage and baby bok choy and cook for 3 to 5 minutes or until the vegetables reach the desired tenderness. Then, take the pot off the heat.
Season the soup with black pepper. Taste to see if you need any additional seasoning.
Ladle the soup into bowls and garnish with sliced scallions if desired. Finally: slurp it up!