Boil the potatoes till soft enough to mash ( 15-20 minutes).
While the potatoes are boiling, blend the rosemary, walnuts and olive oil in a processor. Salt to taste and set aside.
Once the potatoes are cooked, strain and mash them. I prefer to use a masher - it retains the potatoe's natural texture.
Add the butter and blend with potatoes.
Add a litle cream. Depend on the consistency you like, add more cream.
Blend in the rosemary sauce with a fork keeping 3 tablespoons aside for decoration.
Salt to taste.
Transfer mashed potatoes to serving dish.
Decorate top with dollops of the remaining sauce and with a small fork, slowly spread the sauce in circles.