In a mixing bowl, combine 1/2 cup of water or beef stock, vinegar, tomato paste, and spices.
Put the London broil inside and cover. Marinate for 3 hours or overnight in the fridge.
Remove the beef from the marinade and pat dry.
On the Instant Pot, press "saute" and add oil.
Add the beef and sear on all sides until brown.
Pour in marinade and that one extra cup of water or stock.
Lock the lid and cook at high pressure for 20 minutes.
Once done, wait for a natural pressure release. When the valve drops (means all the pressure has been released), open the lid and take out the beef.
Season with salt and pepper and serve.