I roast a whole head of garlic first. Cut the top off the head of garlic, no need to peel. Wrap in aluminum foil up to where you cut the head off and drizzle olive oil over the top. Roast at 400 degrees F until the cloves are soft when you squeeze it, usually 20mins. After it cools, you squeeze all the garlic pulp out of the head and stir that into your mashed potatoes. The roasting gives the garlic a wonderful flavor and takes the sharpness and bitterness out of the garlic.