Preheat the oven to 350°F. Mash the boiled potatoes with heavy cream and butter and season with salt, pepper, and nutmeg. Transfer a little more than half of the mashed potatoes into a greased springform pan and spread over the bottom and the sides.
Brown the ground beef, add the onion and bell peppers, and season everything with salt. Stir in the BBQ sauce and let it come to a boil. Transfer the smoky, spiced beef mixture to the springform pan and spread over the mashed potatoes.
Arrange the cheese slices in a circle on top of the BBQ beef filling.
Dice and fry the bacon, mix it into the remaining mashed potatoes, spread it over the cheese, and smooth it down.
For the creamy topping, combine the sour cream and mayonnaise and spread it as a final layer on top of the hearty pie. Transfer the pan to the preheated oven and cook for 45 minutes. Carefully remove the cooked dish from the springform pan before serving.