Place potatoes and garlic cloves in a 4-quart instant pot, cover with water, and sprinkle salt over top. Fasten lid into place, close vent, and cook potatoes at high pressure 4 minutes.
Quick-release steam from instant pot and remove lid. Drain liquid from instant pot and transfer potatoes to a large bowl. Cover bowl with a kitchen towel to allow potatoes to dry out a bit. Transfer garlic to a cutting board and mash with a fork.
While potatoes are drying out, set instant pot to BROWN/SAUTE function. Add butter and swirl to coat bottom of instant pot. Add kale or cabbage to instant pot and sauté until greens are tender and wilted. Add green onions and cook 1-2 additional minutes until fragrant. Turn off instant pot.
Mash potatoes with a potato masher then transfer to instant pot and fold into sautéed greens with mashed garlic, heavy cream and just enough milk until potatoes are creamy. Season colcannon with salt and pepper to taste and serve warm (colcannon may be kept in instant pot on 'warm' setting up to 2 hours). Enjoy!
Notes
Storing InstructionsStore colcannon in refrigerator up to 6 days.