Select the Sauté setting on the Instant Pot and when the display reads HOT, add the olive oil. Add the ground lamb (or beef) and sauté until cooked through.
Using a slotted spoon, remove the cooked meat and place in a bowl and set aside. Reserve a tablespoon of the fat in the liner of the pot and discard the rest.
Add the onion, celery, carrot, garlic, and 1/2 teaspoon of the salt to the pot and sauté until the onion is translucent, approximately 4 minutes. As this mixture cooks, scrape the bottom of the liner to remove any stuck bits from browning the meat.
Stir in the thyme, pepper, tomato paste, and Worcestershire sauce. Cancel the Sauté cooking mode.
Return the cooked meat to the pot and add the vegetable broth and peas.
Place a tall trivet in the pot. Place the potatoes on the rack. Hint – if they are large potatoes, be sure to cut them in half horizontally for proper cooking.
Lock the lid in place the turn valve to the sealing position. Pressure Cook/Manual on HIGH for 15 minutes.
When the cooking time ends, perform a quick pressure release by slowly moving the valve to the venting position to release the steam.
Carefully remove the lid and transfer the potatoes to a bowl, set aside.
If the meat mixture seems too thin, place the pot on saute mode and let the liquid evaporate as you mash the potatoes.
To the potatoes, add the milk and 3/4 teaspoon salt and use a potato masher to mash the potatoes until smooth. Add additional salt and pepper to taste.
Stir 1/2 cup of the mashed potatoes into the meat mixture in the pot.
Transfer the meat mixture to a broiler-safe 8 inch baking dish. Spread the mashed potatoes on top. Using the tines of the fork, make ridges along the top surface of the potatoes.
Broil the shepherd’s pie until the potatoes are lightly browned. Approximately 5 minutes.