In a food processor, grind the walnuts, cinnamon, and nutmeg and pulse until the walnuts are coarse. Add the dates and nut butter, and pulse until the mixture forms a pliable dough.
Press the walnut mixture into 4 mini tart pans, or one 9 inch pie or tart pan. For the filling, melt the chocolate and cream in a double broiler and stir until melted.
You can also use the microwave by setting it at 30 seconds, stirring, then microwaving 30 seconds more. Let the mixture cool about 10 minutes, then pour over the date walnut crust. Top with the slivered almonds and refrigerate at least 2 hours to set.