Preheat oven to 375°F. Grease a two quart casserole dish, and set aside.
Cook the egg noodles according to package directions, except reducing the cooking time slightly. Drain the noodles when they are just shy of al dente. Set aside.
Heat the butter in a large skillet with high sides or a Dutch oven over medium-high heat until melted. Add in the celery, onion, green pepper, and mushrooms and sauté until tender, about 10 minutes.
Stir in the flour, and cook for an additional 2-3 minutes to reduce the raw flour flavor. Then slowly add in the milk, stirring constantly during addition. Bring to a simmer, and cook until thick and bubbly, about 10 minutes.
Remove sauce from heat, and add in the ground mustard, salt, pepper, and cheddar cheese. Stir until the cheese is melted. Taste sauce for seasoning, adding more salt and pepper if desired.
Add in the cooked noodles, mayo, tuna, frozen peas, and pimento peppers, if using.
Pour mixture into the prepared casserole dish, and set aside.
In a small bowl, combine all the topping ingredients until well-mixed. Sprinkle over top of the casserole.
Bake uncovered in the preheated oven for 20-25 minutes, or until the casserole is bubbly and the topping is golden brown. Remove from oven and let stand for 5-10 minutes before serving.
Notes
Make sure to shred your own cheese for the best results. Pre-shredded cheese is coated in an anti-caking agent, which can make the sauce gritty or or clumpy.
We’ve made this with chickpea pasta before (in particular, Banza Rotini) with good results. Chickpea pasta is a lot more fragile, so just barely cook the pasta before adding to the casserole (2-3 minutes), and be aware that reheating leftovers might result in the pasta falling apart—which looks bad, but still tastes delicious!
In place of panko topping, sometimes we sprinkle on toasted slivered almonds for a wonderful crunch!