Put the potatoes in a pot with salted water to cover. Bring to a simmer over medium heat and cook until tender all the way through, about 10 minutes. Drain.
Combine the butter, heavy cream and 1 1/2 teaspoons salt in the same pot you cooked the potatoes in and melt over low heat. Use a masher to puree the potatoes right back into the pot while still hot. Stir in the sour cream. Add large handfuls of cheese and stir and fold the potatoes over themselves until the cheese melts, then add another handful. If it gets too thick and hard to stir before you've added all the cheese, add a few tablespoons of additional cream. Season with salt and pepper. Serve and add the Gravy on top.
Heat the drippings in a small saucepan (or the meatloaf pan) over medium heat. Whisk in the flour until smooth. Cook and stir until lightly toasted, 1 to 2 minutes. Stir in the thyme. Whisk in the beef stock until smooth. Bring to a simmer over medium heat and stir in the Worcestershire. Simmer until thickened, 4 to 5 minutes. Season with salt and pepper.