Turn the Instant Pot on the saute setting and melt the butter. Brown both sides of the London broil. Remove each browned piece of the roast when it is browned and place it in a bowl.
Pour the water into the bottom of the Instant Pot. Add the two envelopes of powdered onion soup mix. Use a wooden spoon to dissolve the onion soup in the water. Mix until all lumps have dissolved. At the same time scrape the bottom of the pot to loosen any stuck-on pieces from browning the meat.
Place a trivet or a silicon sling into the Instant Pot. Place the pieces of beef on top of the trivet in a single layer if possible.
Drain the potatoes in a colander. Add the drained 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.
Sprinkle with salt and pepper. Add a dried bay leaf and a sprig of fresh rosemary or ½ a teaspoon of dried rosemary. Twist the lid on the pressure cooker. Make sure the valve is set to the sealing position.
Press the manual button and set the timer to 30 to 40 minutes. 40 minutes for a very thick cut of beef.
Allow the meat and vegetables to pressure cook. Once the timer has gone off quick release the steam. When the steam is finished releasing carefully remove the Instant Pot lid away from your face to prevent a steam burn.
Remove the vegetables from the inner pot and place them in a serving bowl. Remove the meat from the Instant Pot and place it on a serving platter. Allow the meat to rest 5 to 10 minutes before slicing or shredding the meat.
While the meat is resting mix the cornstarch with the cold water until lump-free.
Turn the Instant Pot on the saute setting. Bring the juices that are left in the inner pot to a simmer. Pour the cornstarch slurry into the liquids. Stir to combine. Allow the gravy to come to a simmer occasionally stirring. Simmer until the gravy has reached the desired thickness.
Carefully using pot holders lift the inner pot from the appliance and pour the gravy into a gravy boat or bowl. If you want to remove the bits of onion from the gravy strain it through a fine-mesh strainer into a bowl.
Makes 4 to 6 servings.