Sauté the onion, carrot, red pepper and celery in butter until onion is translucent and soft.
In a large bowl, mix the cooked vegetables with cheddar cheese soup, sour cream and milk. Add 1 cup shredded cheddar cheese. Add mushrooms and peas. Season with a little salt & pepper. Stir in the cooked pasta and then gently fold in tuna.
In a 9 x 13 casserole dish (sprayed with non-stick cooking spray), pour pasta and tuna mixture. Top with remaining 1 cup of shredded cheddar cheese. Cover with aluminum foil.
Bake at 350-degrees for 45-55 minutes or until bubbly. Top with crushed goldfish crackers and return to oven for 10 minutes. Remove from oven and let stand 5 minutes. Serve.
(You can use the remaining goldfish crackers to top the casserole with. Kids will love it!)