Preheat the oven to 350 degrees F. In a large saute pan, melt 3 tablespoons of the butter. Add the sugar, 1/2 cup of the brown sugar, lemon juice and 1 tablespoon of the flour. Stir the mixture for 1 minute, to dissolve the sugars. Add the rhubarb and cook for 4 to 6 minutes. Add the apples and continue to saute for 3 minutes. Add the Calvados and carefully flame the apples. Saute for 1 minute. Season the mixture with nutmeg, cinnamon , and salt. Mix thoroughly and remove from the heat. Cool the mixture. In a mixing bowl, combine the remaining butter, brown sugar, flour and walnut pieces. Using your hands, blend well, until the mixture resembles a crumb-like texture. Pour the rhubarb and apple filling into the pie shell. Sprinkle the crumb filling evenly over the fruit filling. Place the pie in the oven and bake for 45 minutes. Remove the pie from the oven. Let the pie cool before serving, about 10 minutes. Slice and serve warm with vanilla ice cream.