Scrub potatoes under cold running water. Bake potatoes until tender, but still slightly firm, about 1 hour. Cool completely, cut into 1-inch cubes and set aside. Combine diced potatoes, water and cider in the bowl of a planetary mixer. Stir in the combined flour, yeast, salt and fennel seeds and mix on low speed. You may prefer to mix this dough by hand. It will be wet and sticky.