Peel and cut the russet potatoes into evenly-sized chunks (about 2 inches) so they cook uniformly. Place the potatoes in a large pot and cover with cold water by about 2 inches. Add 1 tablespoon of kosher salt, the unpeeled garlic cloves, bay leaf, and thyme sprigs. Bringing potatoes to a boil in salted water from the start helps them cook evenly and absorb seasoning throughout. Bring to a boil over high heat, then reduce to a simmer and cook for about 20 minutes until the potatoes are completely fork-tender.

Drain the potatoes in a colander and let them sit for 3 minutes—this allows excess moisture to evaporate, which is crucial for a creamy rather than gluey casserole. Remove and discard the bay leaf and thyme sprigs. The garlic cloves will be soft enough to slip easily from their skins; peel them now and set aside. I find that letting the potatoes rest slightly also makes them easier to mash smoothly without overworking the starches.

Transfer the warm potatoes and peeled garlic cloves to a large bowl and begin mashing with a potato ricer or masher until you reach your desired texture (I prefer leaving it slightly chunky rather than completely smooth). In a separate small bowl or measuring cup, warm the 3/4 cup of milk slightly to help it incorporate smoothly into the warm potatoes. Add the softened cream cheese, 6 tablespoons of unsalted butter, and sour cream to the mashed potatoes, then gradually pour in the warm milk while stirring gently until the mixture is creamy and well-combined. Season with kosher salt and freshly ground black pepper to taste.

Butter a baking dish (9x13 inch works well) and transfer the creamy potato mixture into it, smoothing the top with a spatula for an even surface. Dot the top with 2 tablespoons of unsalted butter cut into small pieces—these will melt during baking and create a golden, rich top. Allow the casserole to cool to room temperature, then cover tightly with plastic wrap or foil and refrigerate overnight or up to 2 days. Make-ahead casseroles like this actually develop deeper flavor as they sit, and chilling allows the potatoes to set properly for baking.

Preheat your oven to 350°F about 15 minutes before baking. Remove the casserole from the refrigerator and uncover it. Bake for approximately 1 hour until the top is golden brown and the edges are bubbling slightly. The casserole will heat through completely during this time. Let it rest for 5 minutes before serving—this helps the potatoes set slightly so they hold their shape on the plate.