In a large saucepan over medium heat, add cabbage and 2 cups water. Bring to boil, then reduce heat and allow to simmer, covered, until cabbage has cooked down (about 1/2 the original size) and is tender, about 10-15 minutes. Drain cabbage, but reserve water cabbage cooked in. Transfer cooked cabbage to a bowl, cover, and set aside.
1/2 head cabbage, 8 cup water
In a large stockpot, add cabbage water and potatoes, plus more water so that the top of the potatoes are covered. Bring to a boil over medium-high heat. Reduce heat and allow to cook, uncovered, until potatoes are tender, about 15-25 minutes.
6 medium russet potatoes, 8 cup water
While potatoes cook, add milk, green onions, salt, and pepper to a small saucepan. Bring to a boil over medium heat and allow to simmer for 20 seconds, then remove from heat. Set saucepan nearby.
1 cup whole milk, 1/2 cup green onion, 1 teaspoon salt, 1 pinch black pepper
Once potatoes are cooked, drain excess water and add potatoes to a large mixing bowl. Mash potatoes to desired consistency (having some lumps is okay).
Add cooked cabbage, milk mixture, and butter to mashed potatoes, then gently stir to combine.
2 tablespoon salted butter
Serve colcannon immediately with crumbled bacon, fresh parsley, and butter.