Preheat oven to 400°F. Line two baking sheets with parchment paper and set aside.
Place the unpeeled, whole potatoes into a medium saucepan and fill with cold water. Bring to a boil and cook until almost fork tender. Drain and set aside to cool completely. Once cooled, thinly slice and set aside.
Set a medium skillet over moderate heat. Once hot, drizzle in 1 tablespoon olive oil and add the sausage. Break up with a wooden spoon, cooking until golden brown and crispy, about 8 to 10 minutes. Stir in the crushed red pepper flakes and fennel, if using. Remove from the heat and allow to cool.
Divide the dough into four equal pieces. Keep them covered with plastic wrap or a kitchen towel to prevent them from drying out. Working with one at a time, roll out on a lightly floured work surface to a large round, about 1/4-inch thick. Brush with olive oil and then top half of the dough with 1/4 cup each of fontina and mozzarella and a sprinkle of parmesan. Top with a few slices of potatoes and season with a bit of salt, pepper and rosemary. Top with few spoonfuls of the sausage. Fold in half, bringing one side of the dough over the filling. Seal and crimp the edges of the calzone with your fingers or a fork. Place one of the prepared baking sheets. Continue filling the rest of the calzones in the same way.
There should be two calzones on each baking sheet. This will give the plenty of room while baking. Brush each with the remaining olive oil and make two small slits on top of each with a sharp pairing knife. Sprinkle with parmesan and rosemary. Bake until golden brown, 20 to 25 minutes. Remove from the oven and allow to cool slightly before serving. You can serve them as is or with warmed store-bought (or homemade) pizza sauce for dipping. Enjoy!