Stir water and yeast and let sit in the bowl of a standup mixer fitted with a dough hook attachment for a minute.
Add remaining ingredients and mix on low speed for 3 minutes. Increase speed to medium and knead for 8 more minutes – dough should clean the sides of the bowl but still be a bit sticky on the bottom.
Place dough into a large, lightly greased bowl and cover with plastic wrap. Let rise in a warm place for an hour.
Meanwhile, boil potatoes in salted water until tender and drain. While still warm, mash with butter and stir in cheese. Season to taste and let cool to room temperature.
Turn dough out onto a lightly floured surface. With a rolling pin, roll dough into a rectangle 14-inches by 24 inches.
Spread potato filling with a spatula evenly on half of the dough, leaving 1/2-inch around the edges.
Fold bare half of dough over potato filling and pinch edges to seal (press dough onto potato filling to push out any air pockets). Slide dough onto a parchment-lined baking tray, cover dough with a tea towel and let rise 30 minutes.
Preheat oven to 175°C Dock the surface of he dough with a fork and bake for 40 minutes, until evenly golden brown. While still warm, brush with melted butter or olive oil.
Bread freezes very well.