Prepare the Filling: In a large bowl, combine the ricotta or farmer's cheese, egg, granulated sugar, vanilla extract, and lemon zest. Mix until all ingredients are well-combined and the filling is smooth. If using, stir in the raisins or currants. Set the bowl aside.

Prepare the Phyllo Dough: Lay a large, clean kitchen towel on your work surface. Unroll the phyllo dough carefully and cover it with another clean, damp kitchen towel to keep it from drying out.

Assemble the Strudel: Take one sheet of phyllo dough and lay it flat on the towel. Brush it lightly with melted butter. Repeat this process with 4-5 more sheets of phyllo dough, layering them on top of each other and brushing each with butter.

Add the Filling: Spread half of the cheese filling evenly over the top layer of the phyllo dough, leaving a 2-inch border along the short sides and a 3-inch border along the long sides.

Roll the Strudel: Using the kitchen towel to help you, carefully lift the long edge of the phyllo and roll it over the filling, creating a tight log. Once rolled, tuck in the short ends to seal the filling.

Bake the Strudel: Carefully transfer the roll, seam-side down, onto a baking sheet lined with parchment paper. Brush the top with a final layer of melted butter. Bake at 375°F (190°C) for 30-35 minutes, or until the strudel is golden brown and the filling is set.

Serve: Let the cheese strudel cool on the baking sheet for a few minutes before slicing it. Dust generously with powdered sugar and serve warm.