Peel and cut your five pounds of potatoes into uniform two-inch cubes. Place them in a large pot covered with cold salted water and bring to a boil over high heat, then reduce to medium and simmer for 12-15 minutes until fork-tender. Drain thoroughly in a colander.

While potatoes cook, mince your four garlic cloves finely or roast them for sweeter flavor. Heat the butter gently in a small saucepan with the minced garlic until fragrant but not browned, approximately three minutes.

Return drained potatoes to the hot pot and mash until mostly smooth. Add the garlic butter mixture and stir gently until combined throughout the potato base.

Pour in the heavy cream and whole milk slowly while stirring constantly. This thanksgiving make ahead mashed potatoes crowd recipe requires gentle folding rather than aggressive mixing to prevent gluey texture from developing.

Fold in the sour cream, chopped fresh chives, and fresh thyme until evenly distributed. Season with salt, black pepper, nutmeg, and cayenne pepper, then taste and adjust seasonings to your preference.

Transfer the mashed potatoes to a buttered nine-by-thirteen-inch baking dish. Cover tightly with plastic wrap and refrigerate overnight or up to two days ahead of serving time.

When ready to serve, preheat your oven to 350°F and cover the dish with foil. Bake for 25 minutes until heated through, then remove foil and bake an additional five to ten minutes until the top is lightly golden.