For oatmeal crust: Mix together oats, butter, brown sugar, water, 1 teaspoon cinnamon, ½ teaspoon cardamom, and the salt in a medium mixing bowl. Spread mixture on the bottom of a baking dish (11-3/4-by-7-1/2-by-1-3/4-inches) that is tightly and smoothly lined with NONSTICK foil and has overlapping sides making it possible to lift out the dessert after it is frozen. Bake for 30 minutes, then cool.
For the pumpkin-ice cream filling: In a mixing bowl, stir together the pumpkin and all the spices until blended. Stir in the softened ice cream until blended. Pour over cooled crust. Fold over the overlapping edges of the foil to cover the top, or you can cover the top with plastic wrap. Freeze the dessert until firm, overnight or even a few days if desired.
To serve: Remove dessert from freezer about 10 minutes before serving to soften slightly. While dessert is standing, whip cream until stiff and place in refrigerator while cutting dessert.
To cut dessert, hold the sides of the overlapping foil and carefully lift up dessert and place on flat work surface, then press the foil flat so you can cut the dessert. Using a large sharp knife, cut dessert into 10 or 12 squares (or however many you are serving at this time). Carefully lift up each piece and place on a serving dish. Place a generous dollop of whipped cream on each square and dust with nutmeg. Serve immediately. Cover and return any leftovers in pan to freezer. Serves 10-12. Note: Do not skip the whipped cream dusted with nutmeg on top for serving. It won't be the same without it!
Recipe by The Culinary Cellar at https://theculinarycellar.com/pumpkin-freezer-dessert/