Peel and dice the potatoes. Keep a bit of the skin on if you like rustic mashed potatoes.
Add the potatoes to a large pot of cold salted water. Bring to a boil, and simmer uncovered until the potatoes are fork tender. Drain the potatoes in a colander.
In a large bowl, add butter, créme fraiche, heavy cream, and salt to the potatoes and mix with a hand mixer until creamy. Adjust the salt to taste. If you like extra butter, I won't judge you :-)
Optional crispy fresh sage on top
If you want to add some extra flavor on top, heat a small skillet over medium heat with a drizzle of olive oil. When the pan is heated, add the the fresh sage leaves and fry until they just start to turn a darker green about 30-45 seconds (depending on how hot your pan is). Remove from the pan and place on a plate to drain. They will be crispy so you can break them apart with your fingers once they have cooled.
Notes
Learn an easy way to keep mashed potatoes warm.If you can't find créme fraiche, use 6 oz. of cream cheese mixed with 1/4 cup greek yogurt. Greek yogurt has less moisture and a more tangy flavor than regular yogurt.