Preheat the oven to 350 degrees F. Spray an 8-inch square baking dish with the cooking spray.
Prepare the noodles in accordance with the directions on the package. Mix together the noodles, soup, peas, tuna, milk, cheese, onion, and pepper in a bowl. Transfer the mixture into the prepared baking dish.
Bake, covered, for about 30 minutes. Sprinkle the crackers on top and bake, uncovered, for another 10 minutes.