In a large bowl, whisk together 2.25 cups all-purpose flour and 1/2 tsp salt. Add the 6 tbsp cold cubed unsalted butter and use a pastry cutter or your fingertips to cut the butter into the flour until pea-sized crumbs remain. Mix 1 tsp white vinegar into 6 tbsp ice water. Drizzle about half the ice water mixture over the flour-butter and gently fold until the dough starts to come together. Add remaining water a tablespoon at a time as needed — dough should hold together but not be sticky.

Turn dough onto a lightly floured surface; gently knead 4–6 times to bring it into a smooth ball. Divide into 2 equal portions for two strudels, flatten each into a disk, wrap in plastic wrap and refrigerate at least 30 minutes (up to 2 hours). Chilling relaxes gluten and makes dough easier to roll.

Filling

Prepare apples

While dough chills, combine 6 cups thinly sliced apples with 2 tbsp fresh lemon juice in a large bowl to prevent browning. If using, sprinkle 1/4 cup raisins over apples to rehydrate them slightly.

Mix filling

To the apples, add 1/2 cup granulated sugar, 1/4 cup packed light brown sugar, 1 tsp ground cinnamon, 1/4 tsp ground nutmeg, 1 tsp vanilla extract and 2 tbsp flour (or cornstarch). Add 1/4 cup chopped walnuts or pecans if using. Gently toss until apple slices are evenly coated. Taste and adjust sugar/cinnamon to preference.

Let the mixture sit 10 minutes so the flour absorbs excess juices; if there is excess liquid, drain a small amount off to keep the strudel from becoming soggy.

Assembly & Baking

Preheat oven & prepare pan

Preheat oven to 400°F (200°C). Line a baking sheet with parchment paper.

Roll and shape dough

Work with one chilled dough disk at a time; keep the other refrigerated. On a lightly floured surface, roll the dough into a rectangle about 12x10 inches and roughly 1/8 inch thick. For a traditional thin strudel, use a clean towel: place the rolled dough on a lightly floured kitchen towel, brush the surface lightly with melted butter, then gently stretch working from the center outward with your hands until the dough is very thin and slightly translucent (it may tear slightly — small tears are fine). Aim for roughly 14x12 inches.

Assemble filling

Leave a 1.5-inch border on the long edge nearest you. Spread half of the apple filling in a line along that edge, mounding slightly in the center but not overfilling. Fold the short edges inward about 1/2 inch, then using the towel to help, gently roll the dough away from you to encase the apples into a log. Transfer the seam-side down onto the prepared baking sheet. Repeat with second dough disk and remaining filling.

Finish and egg wash

Brush each strudel generously with 1/2 to 1 tbsp melted butter, then brush with beaten egg to create a shiny crust. In a small bowl, mix 1 tbsp granulated sugar with 1/4 tsp cinnamon and sprinkle over the tops for a crisp sweet crust. If desired, trim ends slightly to neaten or tuck edges under.

Bake

Bake at 400°F (200°C) for 25–35 minutes, rotating the pan halfway through, until the pastry is golden brown and the filling is bubbling (use a small slit to check steam). If the top browns too quickly, tent loosely with foil for the last 10 minutes.

Rest & serve

Remove from oven and brush once more with any remaining melted butter. Let strudels rest on the baking sheet 15 minutes to set the filling. Dust with 1 tbsp powdered sugar just before serving if desired. Slice into 1- to 1.5-inch pieces and serve warm, optionally with vanilla ice cream or whipped cream.