In a large skillet over medium-high heat, melt 1 tablespoon of butter, then add the celery and onions, sauteing for 2-3 minutes until beginning to soften slightly. Add the garlic and mushrooms, then continue to cook another 2-3 minutes until the mushrooms have released some of their juices and the vegetables are softened. Transfer to a plate and set aside.

In the same skillet, melt the butter for the sauce, then sprinkle with the flour and whisk to combine, reducing the heat to medium. Cook for about 1 minute, then slowly whisk in the chicken broth and milk to avoid lumps. Continue to cook and stir at a low simmer, adjusting the heat as needed, until thickened and bubbly, around 2-3 minutes. Add the salt, pepper, parsley, and parmesan cheese, stirring to combine. Remove from the heat and stir in the sour cream, then taste and adjust the salt and pepper, if needed.

Combine the cooked pasta, sauteed vegetables, peas, drained tuna, cheese, and sauce in a large bowl or the pot used to cook the pasta, stirring to coat everything evenly in the sauce. Transfer to the prepared baking dish.

In a small bowl, combine the breadcrumbs and melted butter. Sprinkle over the casserole, then sprinkle the remaining parmesan cheese on top. Bake for 25-30 minutes until hot through. The casserole will be bubbling around the edges and the breadcrumbs should be toasted a golden brown.