Place peeled and cut potatoes in a large pot, cover with cold water, and bring to a boil over high heat. Cook until the potatoes are tender and easily pierced with a fork, about 15 minutes. Drain thoroughly in a colander.
While the potatoes cook, sauté the chopped cabbage and sliced leeks in a skillet with one tablespoon of butter until softened and lightly browned, about 8 minutes. This draws out their sweetness and creates a fragrant aroma.
Return the drained potatoes to the pot and mash them with a potato masher or fork until smooth and fluffy. Add the remaining butter gradually, allowing each piece to melt in before adding more, creating a creamy base.
Pour the warm heavy cream into the mashed potatoes, stirring gently to combine until the mixture becomes rich and velvety. Keep the mash warm over low heat while preparing to fold in the vegetables.
Fold the sautéed cabbage and leeks into the mashed potatoes, mixing gently to distribute their greens evenly throughout the dish. The mixture should be creamy with bits of green for visual appeal and flavor.
Season with salt and freshly ground black pepper to taste, adjusting the seasoning as needed. Optionally, garnish with extra chopped scallions or a pat of butter before serving.
Serve the colcannon warm, ideally in a bowl or on a plate, with a golden, fluffy texture and a colorful hint of green from the cabbage and leeks. Enjoy this comforting Irish classic straight from the stove!