Turn the Instant Pot on and press the Sauté function.
Add the leeks and butter and cook for 4-5 minutes, until softened.
Add the reset of the ingredients. Reserve half of the peas (if using) for later.
Stir everything in the pot, and press the Keep Warm/Cancel button to stop the Sauté process.
Place and lock the lid, making sure the top vent is pointing to Sealing. Press the Manual setting, HIGH pressure and adjust the time to 10 minutes. After 3 beeps the Instant Pot will start to build up the pressure and begin the cooking process. The pressure should take about 10 minutes to build, plus 10 minutes of cooking, so about 20 minutes in total.
In the meantime, add the remaining frozen peas to a bowl of boiling hot water to defrost.
Once the timer goes off, allow a few minutes for the pressure to release naturally; then use a quick release method (pull the top vent lever to Venting, while standing away from the Instant Pot as the steam will shoot up in the air).
Open the lid, stir through and serve topped with the remaining defrosted peas and a little extra lemon zest and black pepper for garnish, if you like.
Most home cooks err by dicing potatoes and chicken into identical dimensions, resulting in starch that dissolves into mush before the poultry finishes rendering. Pressure cooking relies on sealed steam dynamics rather than direct radiant heat, meaning that denser root vegetables require significantly more structural integrity to survive the pressurized moisture cycle without collapsing entirely. Cutting potatoes into generous cubes measuring roughly twice the volume of the chicken pieces allows both components to finish cooking at the same rate inside the enclosed vessel. When using a multi-cooker like an Instant Pot, ensure the silicone sealing ring is properly seated inside the lid rim; a warped or displaced gasket will vent steam prematurely and prevent the machine from reaching the necessary internal operating pressure of approximately 15 pounds per square inch. Sautéing the leeks in butter first builds foundational savory depth, but completely scraping up any browned bits from the pot bottom before securing the lid is mandatory to avoid triggering the dreaded burn notice. Determining whether dense starches and protein are fully cooked without probing them with a digital thermometer relies on tactile and visual cues. Piercing a large potato cube with the tip of a paring knife should yield zero resistance to the center, while the surrounding chicken must show opaque, white interiors with no residual pinkness when split open. Reserved frozen peas added at the very end require only a brief soak in hot water to revive their bright green color and crisp bite without overcooking. Leftovers store safely in an airtight container in the refrigerator for up to three days.