Preheat oven to 400℉ (205℃).
Line your baking sheet with parchment paper or foil for easier cleanup, then place the potatoes on the baking sheet. Spray or rub them with oil.
Then cut the top ¼-½" off a whole garlic head, exposing every clove.
Place the head of garlic on foil paper together with the thyme. Drizzle it with about a teaspoon of olive oil. Wrap it up, place it in the oven with the potatoes, and roast for 30 minutes or more.
Remove the garlic from the oven and leave the potatoes for another 30 minutes. The garlic cloves should be soft, fragrant, and golden. Discard the thyme, let the garlic cool, and then squeeze the individual cloves out of the skin.
After the potatoes bake for about an hour, they should be soft and cooked through. Let them cool for about 5 minutes.
Carefully slice potatoes in half lengthwise. Gently scoop the insides out with a spoon without destroying the skin.
Run the potatoes and garlic through a ricer into a large mixing bowl or mash them with a potato masher. Add the sour cream, cream cheese, butter, and milk and mix well. Season to taste with salt and pepper. Adjust the texture with more milk as desired.
Add optional ingredients like green onions, chives, and cheddar cheese.
Mix well and spoon the mashed potatoes back into the potato skins. You may have to pile high because mashed potatoes take up more room.
Top each potato with your choice of grated cheese and bake until the potatoes heat through (15-20 minutes).
Remove from the oven and garnish with minced parsley and chives. Or serve the toppings on the side so everyone can customize their own potato.