Place a rack in the center of the oven and preheat to 350 degrees. Bring a large pot of salted water to a boil over high heat. Add the pasta and cook for ½ of the amount of time indicated on the package directions. Drain thoroughly then set aside—the pasta will finish cooking in the oven.
Meanwhile, place a large skillet over medium heat and add the oil. Once the oil is hot, add the celery and onion and cook for 3–4 minutes or until the onion is translucent. Add the mushrooms and cook for an additional 3 minutes—just long enough to soften the mushrooms a bit.
Sprinkle the flour over the vegetables and stir until all of the flour has been moistened. Cook for 2 minutes, stirring constantly. Add chicken broth and stir to make sure all of the flour is dissolved. Stir in milk, garlic powder, mustard powder, salt and pepper, bring the mixture to a bubble then reduce the heat to medium-low. Simmer the sauce for 5 minutes.
Remove the sauce from the heat, add the shredded cheese then stir until completely melted and well combined. Fold in the tuna, peas and cooked pasta. Transfer to a 3-quart casserole dish and sprinkle with breadcrumbs. Bake 25–30 minutes or until the center is hot and the edges are bubbling. Allow the casserole to cool 5 minutes before serving. Enjoy!
One Last Thing
If you like a little spice, try adding a dash of Hungarian paprika or red pepper flakes—yum!