Preheat oven to 350 degrees F (175 degrees C). Spray a 9x13-inch baking dish with cooking spray.
Mix asparagus, onion, flour, Parmesan cheese, and parsley together in a bowl.
Whisk 1/2 cup vegetable oil, eggs, water, salt, and pepper together in a separate bowl; pour over asparagus mixture. Stir asparagus mixture until well combined; spread in the prepared baking dish.
Bake in the preheated oven until a knife inserted into the center of the torte comes out clean, about 45 minutes.
