01 - Preheat the oven to 350°F. Grease a 9x13-inch baking dish with butter or nonstick spray.
02 - In a large mixing bowl, whisk together the pumpkin purée, eggs, evaporated milk, granulated sugar, cinnamon, nutmeg, ginger, cloves, salt, and vanilla extract until completely smooth and well combined. Pour the filling into the prepared baking dish and spread it into an even layer.
03 - In a separate bowl, stir together the rolled oats, all-purpose flour, packed brown sugar, and cinnamon. Pour in the melted butter and mix until the mixture becomes crumbly and the dry ingredients are evenly moistened. Fold in the chopped pecans if using.
04 - Sprinkle the crisp topping evenly over the pumpkin filling, covering the surface. Bake for 40 to 45 minutes, or until the filling is set and no longer jiggles in the center, and the topping is golden brown.
05 - Allow the dessert to cool for at least 20 minutes before slicing. Serve warm or chilled, optionally garnished with whipped cream or a scoop of vanilla ice cream.