Dice the onion, carrot, and celery into uniform 1/4-inch pieces, mince the garlic clove, and peel and cut the potatoes into 1-inch chunks. This preparation ensures even cooking and allows you to work efficiently once you start cooking. Having everything prepped and ready (mise en place) is essential for the Instant Pot method since you'll be working quickly between steps.

Set your Instant Pot to the sauté function and heat the avocado oil. Add the ground lamb and cook for 7 minutes, breaking it apart with a spoon, until it's browned and no longer pink. Remove the cooked lamb to a plate and drain excess fat from the pot, leaving just a light coating. This browning step develops deep flavor through the Maillard reaction, which is the foundation of this dish's complexity.

In the same pot with the residual fat, add the diced onion, celery, carrot, and garlic with 1/2 tsp kosher salt. Sauté for 4 minutes, stirring occasionally, until the vegetables begin to soften and release their aromatics. Then add the tomato paste, thyme, black pepper, and Worcestershire sauce. Stir well and cook for 1 minute to bloom the tomato paste and spices—this caramelization adds richness to the filling.

Return the browned lamb to the pot and stir in the vegetable broth and frozen peas. Place the steam rack inside the pot and arrange the prepared potato chunks directly on the rack. The potatoes will cook above the liquid, steaming in the pressurized environment. Close the Instant Pot lid, set the valve to sealing, and cook on high pressure for 15 minutes. I find that cooking the potatoes separately on the rack this way keeps them from absorbing too much of the savory liquid, which helps them mash smoothly.

Once the cooking time is complete, carefully release the pressure using the quick-release method. Transfer the cooked potatoes from the steam rack to a bowl. Add the warmed milk and 3/4 tsp kosher salt, then mash the potatoes until they reach your desired smoothness—I prefer leaving them slightly chunky for texture. Taste and adjust seasoning as needed.

Remove the steam rack from the pot with the lamb mixture. Stir 1/2 cup of the mashed potatoes into the lamb filling to create a cohesive base layer. Transfer the lamb mixture to a broiler-safe baking dish and top with the remaining mashed potatoes, spreading them evenly. Place under the broiler for 5 minutes, watching carefully, until the top is lightly golden and slightly browned. This finishing step adds a textural contrast and visual appeal.

Remove the shepherd's pie from the broiler and let it rest for 2-3 minutes before serving. This short rest allows the layers to set slightly, making plating cleaner. Serve warm directly from the dish.