To make the filling, put 2 cups of the nuts, 2 cups of the cream, 1 cup of the maple syrup, and a pinch of salt in a heavy saucepan. Cook over low heat until the cream is greatly reduced and the nut mass is the consistency of bread dough. For the last 10 minutes, stir continuouslyso that the nuts and cream do not burn. Let the nut cream cool to lukewarm.
Make the strudel: Put the remaining 1 cup nuts, the sugar, and a pinch of salt in a food processor. Pulse to puree the nuts only to the point at which they are ⅛-inch nuggets. Do not overprocess to a paste.
Lay out a sheet of phyllo on parchment paper larger than the sheet of dough and paint the edges of the dough with the warm melted butter. Then paint the whole sheet. Sprinkle with the nut-sugar mix. Then lay another sheet on top of that, paint the edges with more butter, sprinkle with the nut-sugar mix, and keep going until you have used up 8 sheets of phyllo. With each sheet, start 1 inch from the previous edge in the direction away from you, so that when the strudel is folded up, the edge will hold together better. Then spread the nut cream evenly over the final sheet, keeping the filling 1 inch away from the edges.
Roll the strudel up away from you as tightly as possible, as you would a jelly roll, continuing to butter as you roll. Put aside for a moment with the edge side down and paint a final sheet with butter. Put the roll inside this sheet with the edges down and roll up again, continuing to butter as you roll, so that the ragged seam edge is rolled up inside the new sheet, and you have only one seam.
Preheat the oven to 400°F.
Let the roll cool, then cut it into ½-inch-thick slices. Put the slices on parchment paper on a cookie sheet and bake for 10 minutes. Reduce the oven temperature to 350°F, brush the slices with the remaining melted butter, and bake them for another 10 minutes, or until they are cooked and flaky throughout.
Serve while still warm, half-covered with whipped cream, a drizzle of maple syrup, and more chopped hazelnuts, if you like.