Put the peeled and quartered potatoes in a saucepan, cover with cold water and bring to the boil. Reduce heat and simmer gently for 25 minutes until cooked. Drain well and place in a bowl.
In the meantime, heat the oil in a pan over low heat, add the garlic, brown lightly for 2 minutes, turning occasionally. Add the garlic to the potatoes in the bowl.
Mash the potatoes and garlic with a potato masher. Add the sour cream or sour cream, slowly pour in the warm milk and continue to mash the potatoes. Add the vinegar and season with salt and pepper. Serve immediately.
Variation: Add 6-10 tablespoons of melted butter to the mashed potatoes.