Preheat the oven to 400°F.

Toss the garlic cloves in olive oil and place in a small baking dish or pie plate. Season with salt and pepper. Place in the oven until lightly brown and soft, about 45 minutes, or cook in the microwave for 1 to 2 minutes.

Place the potatoes in a medium-sized pot with the garlic cloves. Add water to cover. Bring the water to a boil, reduce the heat to a simmer, and cook the potatoes until very soft, about 20 minutes.

Remove from the heat and drain. Return the potatoes and garlic to the pot over low heat and add 1/2 cup of the hot chicken stock. Mash the potatoes with a mixer or a potato masher, adding just enough more stock to get the desired consistency. Season to taste with chopped rosemary, salt, and pepper.

If not serving immediately, cover the surface of the potatoes in the pot with a layer of hot broth to keep a skin from forming. When ready to serve, turn up the heat, stir in the broth, and heat through. Alternatively, put the cooled mashed potatoes in a plastic bag, seal tightly, and refrigerate for up to 3 days. Reheat in the microwave or with additional hot broth in a pot.