Preheat the oven to 350 degrees F (175 degrees C). Grease a 9x13-inch pan.

Mix flour, rolled oats, brown sugar, and 1/2 cup butter together in a bowl; transfer and press into prepared pan. Bake until golden brown, 20 minutes.

Beat egg whites in a large deep metal bowl until soft peaks form; add pumpkin, evaporated milk, sugar, salt, cinnamon, ginger, and clove. Pour over baked rolled oats crust. Bake until firm, 30 minutes.

Mix chopped walnuts, brown sugar, and 2 tablespoons butter together; sprinkle over pumpkin mixture and bake for an additional 10 to 15 minutes. Cool. Cut into squares and top with whipped cream if desired.