Put potatoes and onions in a large pot and fill with water until the water just covers the potatoes. Bring to a boil and cook until potatoes are well done. Turn heat down to low. Add in the milk. Use a potato masher to lightly mash the potatoes to desired consistency. Add salt and pepper, adjusting amount to taste. Add butter, allow to melt and stir to incorporate well. Let soup rest 10-15 minutes to allow it to thicken a little before serving. If soup is too thick, add milk or cream to thin to desired consistency.