
Preheat oven to 375°F (190°C). Spray with cooking spray a 9x13 inch baking dish.
Bring a large pot of water to a boil. Cook pasta for about 10 to 12 minutes or until al dente; drain and rinse under cold water.
In a small skillet, over medium heat, add the 2 tablespoons of butter. When the butter has melted, saute the ¼ cup onions until soft and translucent. Remove skillet from heat and set aside.
In a large bowl, mix all ingredients except the noodles (Note: reserve ½ cup of cheese to add on top of the casserole). Add the noodles and mix thoroughly.
Transfer mix to the prepared baking dish and sprinkle the remaining 1/2 cup of shredded cheese. Bake for 20 minutes or until the cheese is completely melted.
Serve while hot.