Set pressure cooker to sauté function and wait for it to warm up. Rub oxtail with olive oil and season generously with salt and pepper. Brown the oxtail in batches on all sides until golden brown. (You can also do this step in a 220˚C oven ona large baking tray) Remove oxtail from inner pot and set aside. Add in sliced onions and sautée until soft.Add garlic and cook for 1 minute. Add tomato paste, soy sauce, bay leaves,cloves and thyme. Cook for a few minutes until the tomato paste smells sweet.Add in the beef stock and make sure to deglaze the bottom of the pot very wellso nothing is stuck to the inner pot. Return the browned oxtail to the inner pot.Ensure the pieces are partially submerged in the liquid. Finally, layer the carrots on top. Seal the pressure cooker and make sure the venting knob is in the sealing position. Pressure cook on ‘High’ for 40 minutes and then natural release for 20 minutes. Release any remaining pressure manually and open the lid. Very gently and carefully remove the oxtail and carrots with a slotted spoon and set aside. Discard the thyme sprigs. Put the pot back on sauté mode and bring the sauce to a simmer. Add in the butter beans and allow the sauce to reduce. In the meantime cook the mushrooms. Heat a large frying pan on high heat. Add the olive oil and butter and sauté the mushrooms until golden brown. Just before finishing add the garlic and then season well. Carefully place the oxtail and carrots back in the sauce and beans to keep them hot.Top with the golden brown mushrooms. Scatter everything with fresh thyme and serve on steamed white rice.