Soak raisins: The night before, mix the raisins and rum. Cover and soak overnight at room temperature.
Make dough: Sift the flour and mix with the water, oil, and salt. Knead on the counter for about 10 minutes until smooth.
Rest: Shape into a ball, brush with oil, cover, and rest at least 30 minutes.
Toast breadcrumbs: Melt the butter in a skillet, add the sugar and breadcrumbs, and toast until light golden. Cool.
Make filling: Toss the apples with lemon juice. Mix in the breadcrumbs, cinnamon, and raisins (plus walnuts and sour cream for the old-fashioned version).
Preheat: Heat the oven to 420 to 445°F (conventional bake). Line a baking sheet with parchment.
Roll and stretch: Dust a large cloth on a table with flour. Roll the dough out, then stretch it from the center with the backs of your hands until paper thin. Pull the edges over the table and trim any thick edges.
Butter and fill: Brush the dough with melted butter. Spread the filling in a row along the bottom edge.
Roll up: Fold in the short ends, then lift the cloth to roll up the strudel completely.
Bake: Place seam side down on the baking sheet, brush with melted butter, and bake 30 to 35 minutes until golden.
Serve: Cool until warm, dust with powdered sugar, and serve with vanilla whipped cream.