1. Preheat the oven to 375°F and line a baking sheet with parchment. Unroll the thawed 10 phyllo sheets.
2. Prep the apples: Toss the sliced 900 grams tart apples (Granny Smith or Honeycrisp), peeled and thinly sliced apples with 80 grams granulated sugar (for apples) sugar, 1.5 teaspoons ground cinnamon cinnamon, the 1 lemon, zested and juiced lemon zest and juice. Let sit 10 minutes, then drain off any liquid so the filling stays dry.
3. Make the mascarpone cream: Gently stir together 250 grams mascarpone, cold mascarpone, 40 grams granulated sugar (for cream) sugar, 1 egg yolk egg yolk, and 1 teaspoons vanilla extract vanilla until just smooth. Don't overmix or it can turn loose.
4. Layer the phyllo: Lay one phyllo sheet on parchment. Brush lightly with 100 grams butter, melted (for brushing) melted butter, using about a fifth of it in total for the layering. Repeat with the remaining sheets, and gently press the stack together.
5. Layer the mascarpone cream and apples on top. Fold in the short sides, then roll up snugly from the filled edge, ending seam-side down. Slide onto the baking sheet using the parchment.
6. Brush the whole strudel generously with the remaining butter. Cut a few shallow diagonal slits across the top to let steam escape. Bake until deep golden and crisp, 30 to 33 minutes, brushing with any leftover butter halfway through.
