Wash, peel and slice the potatoes into one-inch chunks. Place in a large saucepan and fill with water to cover the potatoes.
Bring a large pot of water to boil. Add in a teaspoon of salt.
Boil at a low boil for 20-25 minutes, or until potatoes are fork-tender, but not falling apart.
Remove from heat and drain. Place in a large bowl.
While the potatoes are still hot, use either a potato masher or a potato ricer to roughly mash the potatoes in the large bowl. You don't have to mash them completely right now!
Place your butter and cream into a glass measuring cup and microwave in short bursts on high heat until the butter is melted and the mixture is steaming. Do NOT boil.
Pour the butter mixture over the mashed potatoes and mash into the potatoes again. (this is why you don't fully mash them easier on.) Stir in any of your optional add-ins at this point as well.
Grease a 9x13 pan and spoon the potatoes into the pan, smoothing out into one layer. Cover tightly with plastic wrap and refrigerate for up to 48 hours.
To Bake
Remove the casserole and let it come to room temperature.
Preheat your oven to 350 °F.
Melt the 1/4 cup of butter and drizzle over top of the mashed potatoes in the pan. Bake in the preheated oven for 40-45 minutes until hot in the middle and the top has browned slightly.
Garnish with fresh parsley.
Notes
I use ALL of the add-ins for this recipe, but you can pick and choose what you prefer!