Preheat oven to 375 degrees F and butter a 9x13 baking dish. Set aside.
Cook pasta in salted water according to directions.
Meanwhile, melt 2 tablespoons butter in a large skillet. Add the diced onions and celery; sauté until tender (about 3 minutes).
Add the minced garlic; sauté until fragrant (about 30 seconds).
Add the mushrooms, stir with the other veggies, then allow to cook until most of the liquid has evaporated, stirring only occasionally (about 10 minutes).
Add the flour (1/4 cup), stir to coat, and gradually add the milk (1 cup) and broth (1 cup). Heat over medium-high heat until thickened, stirring frequently. Season with salt and pepper to taste.
Meanwhile, in a small bowl, melt remaining tablespoon of butter. Mix with breadcrumbs (1/2 cup).
Add the cooked, drained pasta and tuna to the mixture. Spread mixture into the prepared baking dish. Sprinkle breadcrumb mixture on top. Top with the shredded cheese (1 cup).
Bake for about 20 minutes, or until browned and bubbly.
Notes
You can make your own whole wheat breadcrumbs by pulsing some toasted bread in a food processor until bread crumbs are formed. Store bought is fine too :-)
Make it without milk by using 1 more cup of chicken broth instead.
Frozen peas can be added to the skillet with the tuna and pasta, if you want (about 1 cup).
If you prefer, you can use whole wheat flour instead of all-purpose as a thickener.
Storage: Store covered in foil in your fridge for up to 5 days. Bake covered for about 20 minutes, then uncovered for another 15-20 minutes.
Freeze it: Wrap it tightly in foil or plastic wrap and freeze for up to 3 months before cooking. I recommend letting it defrost completely in the fridge for 24-48 hours before cooking it.
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The provided nutrition information is a best estimate only and is calculated using an automatic API. It does not include any added sodium from seasoning to taste, any optional ingredients, and it does not take brands into account. Read more in my disclaimers page.