Wash and dry potatoes, then peel and dice them. Place into a large pot of cold, salted water and bring to a boil. Reduce heat and cook potatoes until fork tender, about 7-10 minutes.
While potatoes are cooking, bring 3 cups of water to a boil in a medium pot. Add cranberries, reduce heat and simmer for 5 minutes. Remove cranberries with a slotted spoon, place on a paper towel and pat dry. Transfer cranberries to a bowl and gently mix with brown sugar. Set aside.
When potatoes are fully cooked, drain and immediately transfer them to a large bowl. Add milk, butter, 1 tbsp of fresh sage, cinnamon and salt. Mash thoroughly until potatoes are smooth and lump-free. Gently stir in feta cheese and cranberries. Finish by garnishing potatoes with remaining sage. Enjoy!
