Preheat oven to 350º. Butter and lightly flour a baking sheet
In a large bowl, add the flour, sugar, baking powder, and salt, cut in 4 tablespoons of butter with a pastry knife until it resembles coarse meal. Add the milk and stir until dough is smooth and can be gathered into a ball. Place the dough ball onto a lightly floured surface and knead for 2 minutes, then roll out into a 14x11” x 1” thick rectangle.
Toss the rhubarb, apples, walnuts together in a bowl until well blended. Spread the fruit mixture over the rectangle to within 1” of the edges. Sprinkle the remaining 4 tablespoons of butter over the fruit, and sprinkle the brown sugar on top.
Fold the shorter 11” side of the dough over the fruit. Then starting at one narrow end, roll up the rectangle lengthwise to create a tight cylinder about 9” long. Transfer the roll, seam side down, to the prepared pan. Bake for 35-40 minutes or until golden brown.
Serve with whipped cream.