In a pressure cooker, brown roast on all sides in oil. Remove roast. Add cooking
rack; return roast to pan. Add 4 cups of water. Close cover securely; place pressure
regulator on vent pipe. Bring cooker to full pressure over high heat. Reduce heat
to medium-high; cook for 40 minutes. (Pressure regulator should maintain a slow
steady rocking motion; adjust heat if needed). Remove from the heat; allow pressure
to drop on its own. Remove meat and keep warm.
Pour juices into a cup or bowl; skim fat.
Remove rack from cooker. Add potatoes, carrots, onion, garlic, salt, pepper,
thyme, bay leaves, pan juices and remaining water to cooker. Cover securely; return
cooker to full pressure. Reduce heat; cook for 8 minutes. Immediately cool according
to manufacturer's directions until pressure is completely reduced. With a slotted
spoon, remove vegetables and keep warm. Discard bay leaves.
Combine cornstarch and cold water until smooth; stir into the pan juices. Bring
to a boil; cook and stir for 2 minutes or until thickened.