- 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.