6 cups thinly shredded kale, brussels sprouts, or green cabbage
½ cup thinly sliced green onions, white and light green parts only
¼ cup chicken or turkey broth
Kosher salt and freshly ground black pepper to taste
4 pounds small russet potatoes, peeled and cut into 2-inch pieces
4 ounces cream cheese, at room temperature
1 cup whole milk, warmed
Melt 2 tablespoons of the butter in a large stockpot over medium-high heat. Add greens and green onions; stir to coat. Add broth; cover and cook, stirring often, 10 minutes or until tender. Add salt and pepper to taste. Cover and set aside to keep warm.
Bring potatoes and salted water to a boil in a large saucepan. Reduce the heat to medium, and simmer 20 minutes or just until potatoes are tender. Drain the potatoes and let stand 3 minutes or until dry. Return them to still-warm pan. Mash with a potato masher until smooth. Stir in cream cheese and 4 tablespoons of butter. Stir in ½ cup of warm milk.
Fold in the greens mixture. Add up to ½ cup more milk, 1 tablespoon at a time, and stir until mixture is thick and creamy. (Add a little more than you think you’ll need because the mixture will firm up as it chills in step 5.) Season with salt and pepper.
Scrape into a buttered 2½-quart gratin dish or baking dish and smooth the top. Dot with the remaining 2 tablespoons of butter.
Cover and refrigerate overnight. Preheat the oven to 350 degrees. Bake until heated through, about 30 minutes.