Sauté onion in a little bit of oil or butter (which ever you prefer) over medium heat. Until its a nice golden brown, and soft.
Peel and chop potatoes, cover with water (salt the water) and bring to a boil. Then bring it down to a simmer for 15-20 minutes until you can easily poke a fork thru the potatoes. Drain potatoes.
Add in milk (it shouldnt be cold), butter, salt and pepper to taste. Mash all ingredients together with potatoes until you get the consistency you like. Some like super creamy, others like somewhat lumpy. (I think lumps give it more of the homemade feel)