Saute onions in unsalted butter with a drizzling of olive oil until a toasted golden brown and tender, anywhere from 10 - 20 minutes. Add Kosher salt (about half a teaspoon) and pepper to taste. Reserve.
Peel potatoes and then chop into chunks, cover with water plus one inch, and then boil with a pinch of salt. Potatoes are finished when you can easily push a fork into the chunks, approximately 30 minutes at a rolling boil. Drain when done.
Combine drained potatoes and reserved onions. Add salted butter (can add more, to taste), plus cream or milk (less or more to reach desired texture), and Kosher salt and pepper to taste.
Sour cream may be added for texture and taste, as well.
Variant: Real bacon bits, fried crispy, can be added for extra dimension or taste.
Note: I never skimp with the salted butter. I start with half a stick and then keep adding until I'm satisfied, same with the salt added to the potatoes (not onions). I've had a lot of bland mashed potatoes in my day, and salt and butter fixes that, every time.