Roast-beef with sweet potato mash

Ingredients
(serves 4 people)

800g-1kg prepared roast-beef
1/2 teaspoon ground black pepper, or 1 teaspoon smashed black peppercorns
Salt
3-4 sweet potato, peeled and cubed into small pieces
3-4 baking potato, peeled and cubed into small pieces
1 tablespoon butter
3 tablespoons cream fraîche
100ml  (or more if necessary) cooking cream (about 12-15% fat)

Instructions

Cover the roast-beef piece with salt and pepper. Preheat the oven to 225 degrees and then brown the meat for 15 min. Put the temperature down to 165 degrees and let the meat cook until it is done (it has to be about 60 degrees inside). It should take about 40 min.

While the meat is cooking, place the sweet potato and baking potato chunks into a pot of cold water. Bring to a boil, and cook until fork-tender, about 20 minutes from the boil.
Drain off the potatoes, and then return them to the same hot, but now dry, pot. Put the pot on a low heat for just about 1 minute, to allow any excess liquid to burn off. Add in the butter and crème fraîche and season with salt and a bit of pepper. Mash the potatoes with a potato masher. Then add the cream until you have the consistency you desire. Mash a bit more, and you're done.