Cook the pasta according to the package directions.
Add the butter to a large skillet set over medium heat. When melted, add the onion and celery to the skillet. Sauté until softened.
Add the cream of mushroom soup and half & half to the skillet, and stir to evenly incorporate with the sautéed veggies.
Add the shredded cheddar and the cheese sauce packet included with the pasta to the skillet, again stirring to combine.
Add the tuna to the skillet, stirring to evenly incorporate. Remove the skillet from heat, and set aside.
Drain the cooked pasta, then transfer the hot pasta to the skillet. Stir together until everything's combined and creamy.
Lightly spray a small casserole dish with nonstick spray, and then transfer the macaroni and cheese mixture to it. Spread it out into an even layer.
Sprinkle the fried onions evenly out overtop.
Bake uncovered at 350° for 30 minutes. After baking, let the macaroni rest for 5-10 minutes to set up.
Top with fresh parsley leaves and chopped tomatoes, serve, and enjoy!