Preheat oven to 400 degrees F. Spray a large casserole or baking dish with non-stick spray and set aside.
Bring a large pot of water to a boil over high heat. Once boiling, add the noodles and cook until al dente, or approximately 1-2 minutes less than the recommended cooking time. Immediately drain and rinse with cool water. Set aside.
Add olive oil to a large pan (with high sides) over medium heat. Add the onions, carrots, and celery and cook, stirring often, until starting to soften (approximately 8-10 minutes). Add the garlic and cook for an additional minute, stirring continuously.
Once the onion, carrots, and celery are starting to soften, add the frozen peas, chopped mushrooms, Italian seasoning, and salt. Mix thoroughly to combine. Continue to cook until the mushrooms drop their water
Add 4 tablespoons of butter, and allow it to melt. Mix thoroughly with the vegetables. Sprinkle with all-purpose flour and mix until flour is incorporated and no dry parts remain.
Slowly add the milk, whisking continuously to prevent clumping. Once all the milk has been added, add the chicken broth and slowly whisk until incorporated. Mix in the fresh parsley and fresh lemon juice. Allow mixture to come to a low boil before reducing heat to low to keep warm.
Remove from heat and mix in the canned, drained tuna, cooked egg noodles, and shredded parmesan cheese. Season with salt and pepper, to taste. At this point, you may serve your pasta as is, or bake it in a large casserole dish.
To bake, transfer the noodles to your prepared casserole dish. Melt the remaining tablespoon of butter in a small microwave-safe dish and use your fingers to incorporate it with the crushed butter crackers. Sprinkle the casserole with your prepared buttery cracker breadcrumbs.
Bake at 400 ° F for 15-20 minutes, or until the top is golden and the filling is bubbling on the sides.