Brown the London Broil in the Instant Pot on saute setting. Set London Broil aside. Pour water and onion soup mix into the Instant Pot deglazing the pot. Place a trivet or silicon sling in the Instant Pot and put the meat in it.
Add rinsed potatoes on top of the meat around the edges of the pot. Next, add the minced garlic. Place the carrots on top of the potatoes. Add the spices. Place lid on Instant Pot. Set to seal. Press the manual button and set the timer to 30 to 40 minutes. 40 minutes for a very thick cut of beef.
When time is up, quick release pressure. Place vegetables in a serving bowl. Place meat on a serving platter and allow to rest 5 to 10 minutes before slicing. While the meat is resting mix the cornstarch with the cold water until lump-free.
Using the saute setting, bring the juices left in the Instant Pot to a boil. Stir in the cornstarch slurry. Simmer until the gravy has reached the desired thickness. Pour the gravy into a serving dish and serve with the delicious vegetables and tender London Broil.