In a large saucepan, cook 8 ounces pasta of your choice in salted water for slightly less than it recommends on the package (around 2 mins less).
Drain pasta, and set aside.
Meanwhile, add 1 cup breadcrumbs to a small bowl, and add 2 tablespoons butter(melted). Mix until well combined, then add half of the 1 cup grated cheddar cheese.
Pour 1 cup milk and 14 ounce can of cream of chicken soup into a large saucepan, and mix well.
Add diced ½ cup red bell pepper, ½ cup green bell pepper, ½ cup celery, 8 ounces solid tuna, and the remaining cheese.
Heat over a medium heat until JUST beginning to bubble around the edges.
Stir in 1/4 cup fresh parsley if using, then remove from the heat.
Add the pasta to the tuna mixture, and mix until well blended.
Pour into a 8 x 8 casserole dish, and top with the breadcrumbs/cheese mixture.
Bake at 400°F for 30 minutes, or until the topping is golden brown.
Remove from the oven and let it stand for 5-10 minutes.