Cut potatoes into 1.5-inch chunks (no need to peel Yukon Golds—their thin skin adds texture and nutrients). Bring a large pot of salted water to a boil, then add potatoes and simmer for 15-20 minutes until completely tender and easily pierced with a fork. While potatoes cook, finely shred cabbage into 1/8-inch ribbons, mince the garlic clove, and slice green onions into 1-inch pieces, keeping white and green parts separate. Warm the milk in a small saucepan or microwave so it won't shock the hot potatoes when added.

While potatoes finish cooking, melt 2 tablespoons of butter in a large pot over medium heat. Add the shredded cabbage and cook for 3-4 minutes, stirring occasionally, until it softens slightly and begins to caramelize at the edges. Add the minced garlic and white parts of the green onions, cooking for another minute until fragrant. I like to let the cabbage get a little golden—this builds a deeper, more savory flavor than just wilting it.

Drain the cooked potatoes thoroughly and add them directly to the pot with the sautéed cabbage mixture. Pour in the warm milk and begin mashing with a potato masher, working until you reach your desired consistency—I prefer leaving it slightly chunky rather than completely smooth for better texture. Gently fold in the reserved green onion tops, then season with the black pepper and a pinch more salt if needed. The warm milk helps the potatoes mash smoothly without becoming gluey.

Transfer the colcannon to a serving bowl and top with the remaining 1 tablespoon of butter, allowing it to melt into the warm potatoes. Garnish with fresh chopped parsley for color and brightness. Serve immediately while hot and steaming.