How to Make It
Place potatoes, water, 3/4 teaspoon of the salt, and 1/4 teaspoon of the pepper in a programmable pressure multicooker (such as Instant Pot).
Sprinkle salmon on both sides with zest and remaining 1/2 teaspoon salt and 1/4 teaspoon pepper. Place salmon, skin side down, on top of potatoes. Cover cooker with lid, and lock in place. Turn steam release handle to SEALING position. Select MANUAL/PRESSURE COOK setting. Select HIGH pressure for 5 minutes. (It will take about 5 minutes for cooker to come up to pressure before cooking begins.)
Carefully turn steam release handle to VENTING position, and let steam fully escape. (This will take about 3 minutes.) Remove lid from cooker.
Massage kale, lemon juice, and oil together in a large bowl until kale is softened.
Remove salmon from cooker, and set aside. Select SAUTÉ setting on cooker. Select HIGH temperature setting, and allow to preheat. Add kale mixture and garlic to potatoes in cooker; cook, stirring occasionally, until most of the liquid has evaporated, 4 to 5 minutes. Add butter; cook, stirring and smashing potatoes with a wooden spoon, 1 minute. Turn cooker off.
Break salmon into large (1-inch) chunks. Divide potato-kale mixture and salmon pieces evenly among 4 plates.