You can use whatever root vegetables you would like and in amounts you want. 6 to 7 cups of cut up root vegetables are about right for this soup. Saute the onion in the oil until soft. Add the garlic, Thyme and Bay leaves for the last minute, do not burn. Add the oxtails, cut up stew meat and beef broth and stir. Add the pressure cooker lid and bring to pressure according to the instructions for your pressure cooker. When at full pressure, lower heat and cook at pressure for 30 minutes and turn off heat. Important- since this is meat you need to let the pressure drop naturally. Once the pressure drops, gently remove the oxtails and bay leaves. They will be soft, try to remove all of the remaining fat and connective tissue. Remove the meat from the oxtails and return to the pressure cooker. One day or two days? This recipe can be stopped at this point and the pressure cooker moved to the refrigerator to cool until the next day. the fat will congeal on the top and most removed. If you are continuing immediately, use a spoon to skim the majority of the fat from the surface. If doing in 2 days- 2nd day leave the lid off the cooker and bring up to near a simmer (1 day it is still hot). Add the tomato paste, salt and pepper and stir. Add the cut up vegetables. Lock the cover on the pressure cooker and bring to pressure. Once at full pressure, cook for 3 minutes. Immediately release the pressure with the valve (see your pressure cooker manufacturer). (the old way of moving the cooker to the sink and running a small stream of cold water on the lead and side is no longer suggested- your choice though). Taste and adjust the salt and pepper. Optional- garnish with chopped fresh parsley. Serve with good crusty bread!