Wash and dry the produce. Peel and thinly chop both the onion and carrot. Finely chop the celery. Place the onion, carrot, and celery into a medium bowl. Set the bowl aside.
Peel and mince the garlic and finely chop the rosemary. Set the garlic and rosemary aside in a small bowl. Peel each potato and then cut each potato into quarters. Set the potatoes aside in a medium bowl.
Coat the inside of your Instant Pot with cooking spray and then select the sauté function.
Add the chopped onion, carrot, and celery to the Instant Pot and sauté. When the vegetables begin to soften, add the mushrooms. Cook for another 2 minutes before adding the garlic, the rosemary, 2 teaspoons of Worcestershire sauce, the tomato purée, and the ground lamb.
Continue to sauté until there is no visible pink remaining in the ground lamb. Pour in the beef stock.
Place a tall trivet over the ground lamb mixture. Place the potatoes on top of the trivet.
Place the lid on the Instant Pot and set the pressure release to sealing. Turn off the sauté function. Select pressure cook or manual program and set the cooking time to 15 minutes at high pressure.
Conduct a quick pressure release when the cooking is complete. Carefully move the pressure release to venting.
Preheat the oven to broil.
Using a pair of tongs, transfer the potatoes to a large bowl. Add the milk, remaining teaspoon of Worcestershire sauce, and salt and pepper to taste. Mash with a potato masher until the potatoes are smooth.
Remove the tall trivet from the Instant Pot using an oven mitt. Pour the frozen peas into the lamb mixture and add additional salt and pepper to taste. Stir.
Pour the meat into an ovenproof 8- or 9-inch round or square baking dish. Spread the mashed potatoes evenly on top of the meat.
Place the baking dish under the broiler for 5 minutes or until the potatoes are lightly browned. Serve warm. Enjoy!
Tall trivets for your Instant Pot may be purchased on Amazon.