Preheat oven to 350 degrees F. In bowl of food processor fitted with a steel blade combine almonds, brown sugar, and salt. Pulse until coarsely ground. Transfer to bowl and stir in melted butter. Turn into 9-inch pie plate. With fingers press onto bottom and sides to form a firm, even layer.
Bake 8 to 12 minutes until lightly golden. Don't overbrown or the crust will have an overpowering flavor. Allow to completely cool on a rack. Transfer to freezer until ready to use (may be made up to a week in advance and frozen, tightly wrapped).
In large, wide, nonreactive skillet combine blueberries, sugar, cornstarch, and salt, stirring well with a spoon to distribute the sugar and cornstarch. Add zest, lemon juice, water, and a few gratings of fresh nutmeg; stir again to blend.
Cook and stir over medium heat until blueberries begin to pop, give off juice, and come to a full simmer. Simmer, stirring gently for 1 additional minute until sauce is lightly thickened and cornstarch is well cooked.
Set aside to cool; refrigerate sauce until completely chilled before proceeding. (Consistency will be too soft unless sauce is thoroughly chilled.)
Transfer ice cream to a mixing bowl and let stand in the refrigerator 30 minutes or until just softened.
Spoon half into prepared crust. Spread in even layer and top with the chilled blueberry sauce. Spoon on remaining ice cream and spread to edges.
Cover surface of ice cream with plastic wrap and freeze at least 8 hours or overnight until firmly set.
In chilled mixing bowl, whisk crème fraîche, heavy cream, and sugar until just thickened to spreading consistency. Do not overbeat -- it's very easy to go from smooth to grainy. Spread cream mixture over pie; top with blueberries. Serve with heated sauce on the side. Makes 10 servings.