Add the cabbage to a large Dutch oven, fill with enough water to cover the cabbage, plus one inch.  

Place the pot over high heat and bring to a boil.  Cover, reduce the heat to medium and steam the cabbage until very tender – about 20 minutes.

Use a slotted spoon or skimmer to remove the cabbage to a paper towel-lined baking sheet to dry out. Reserve the cooking water.

To the cabbage water, slide in the quartered potatoes.  Add more water if needed to completely cover the potatoes. Cover the pot and boil the potatoes until soft – about 20 – 25 minutes.

Meanwhile, in a small saucepan over medium-low heat, melt the butter.  Toss in the scallions and allow to sweat for about a minute.  

Pour in the half and half and whisk in the sour cream.  Bring to a simmer, but do not boil. Allow to simmer for about 2 minutes.  Remove from the heat and set aside.

Drain the potatoes completely.

Add the potatoes to a large serving bowl.

Using a hand masher, mash the potatoes to the desired consistency.  Stir in the cabbage and cream mixture until well combined and the potatoes are creamy.  Season with salt and pepper. Taste and adjust seasonings, if needed.

Using the back of a spoon, make a dent in the top of the potatoes.  Lay the 2 tablespoons of butter in the indentation.

Sprinkle with the remaining 2 minced scallions and parsley.

Serve immediately.