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Brown the meat: Select the Sauté setting on the Instant Pot and heat the avocado oil. Add the lamb and sauté, breaking it up with a wooden spoon or spatula, for about 7 minutes, until cooked through and no traces of pink remain.Set a colander in a bowl. Wearing heat-resistant mitts, lift out the inner pot and pour the lamb into the colander, letting it drain. Return the inner pot to the Instant Pot housing. (Note from Emma: I just scooped out the meat with a slotted spoon and then poured off all but a tablespoon of fat, which I used for sauteing the vegetables.)
Cook the vegetables and stir in the meat: Add the onion, celery, carrot, garlic, and 1/2 teaspoon of the salt to the pot and sauté for about 4 minutes, until the onion is translucent. (Note from Emma: As the onions release moisture, use that to scrape up any browned bits from the bottom of the pan. More flavor!)Stir in the thyme, pepper, Worcestershire, and tomato paste. Return the lamb to the pot, then add the vegetable broth and peas.
Add the potatoes: Place a tall steam rack in the pot, making sure all of its legs are resting firmly on the bottom. Place the potatoes in a single layer on the rack.
Pressure cook the filling and the potatoes together: Secure the lid and set the Pressure Release to Sealing. Press the Cancel button to reset the cooking program, then select the Pressure Cook or Manual program and set the cooking time for 15 minutes at high pressure. (The pot will take about 15 minutes to come up to pressure before the cooking program begins.)
Vent the pressure: When the cooking program ends, perform a quick pressure release by moving the Pressure Release to Venting.
Mash the potatoes: Using tongs, transfer the potatoes to a bowl. Add the milk and remaining 3/4 teaspoon salt, then use a potato masher to mash the potatoes until smooth. (Note from Emma: If the filling seems a little liquidy at this point, put the Instant Pot on the Sauté function and let it simmer while you mash the potatoes.)Wearing heat-resistant mitts, remove the steam rack from the pot. Stir 1/2 cup of the mashed potatoes into the lamb mixture in the pot. (Note from Emma: Taste both the mashed potatoes and the filling at this point, and add more salt if needed.)
Assemble the casserole: Transfer the lamb mixture to a broiler-safe 8-inch square baking dish, dollop the mashed potatoes on top, and spread them out with a fork, creating a surface texture.Note: If you like, instead of assembling and broiling the pie, you can simply serve the lamb mixture with the mashed potatoes on the side.
Broil the casserole: Broil the shepherd’s pie in a toaster oven (or a conventional oven) for about 5 minutes, checking often, until the potatoes are lightly browned.
Serve: Spoon the shepherd’s pie onto plates and serve immediately.Note from Emma: Leftovers will keep for about a week in the fridge and can be warmed in a low oven or in the microwave.