Preheat the oven to 400°F. Line a large baking sheet with parchment paper., To make the filling: In a large frying pan, melt the butter over medium heat. Add the mushrooms and cook until very tender and soft, 7 to 9 minutes. Add the shallots and fry until tender, 2 to 3 minutes more. Add the salt and pepper, then continue to cook until the mushrooms fully release their moisture and the pan is relatively dry, 3 to 5 minutes more., Add the crême fraiche or sour cream and stir to combine. Bring to a simmer and cook until the mushrooms have absorbed most of the moisture, 8 to 10 minutes. Remove from the heat, stir in the parsley, and set the filling aside to cool to room temperature., To assemble: Stretch the strudel dough as directed in the recipe. Drizzle with 3 tablespoons of the melted butter, then sprinkle with the bread crumbs and distribute the cooled filling on top as evenly as you can. Roll up and trim the strudel as directed, then transfer to the prepared baking sheet., To finish and bake: Brush the top of the strudel with the remaining butter. Bake for 27 to 32 minutes, rotating the pan halfway through the baking time. Remove from the oven and cool for at least 15 minutes before slicing.