1 cup coarse fresh breadcrumbs (see Tips), preferably whole-wheat
¾ cup grated aged Cheddar cheese
1Preheat oven to 375°F. Coat a 9-by-13-inch pan (or similar-size 3-quart baking dish) with cooking spray.
2Bring a large pot of water to a boil and cook noodles according to package instructions. Drain and set aside. Dry the pot.
3Heat butter and 1 tablespoon oil in the pot over medium heat until the butter is melted. Add onion and celery; cook, stirring occasionally, until soft, about 5 minutes. Add mushrooms and cook, stirring occasionally, until their liquid evaporates, 3 to 5 minutes. Stir in sherry, soy sauce, salt and pepper and cook until almost all the liquid has evaporated, 2 to 4 minutes more. Sprinkle flour over the vegetables, stir to coat and cook for 30 seconds.
4Increase heat to high, add ½ cup broth and stir until starting to boil. Add the remaining 1½ cups broth, milk and mustard; bring to a boil and cook, stirring often, until the sauce thickens, about 2 minutes. Add tuna, spinach and the reserved noodles; cook, gently stirring often, until heated through, about 3 minutes. Transfer to the prepared baking dish.
5Combine breadcrumbs and cheese in a small bowl. Stir in the remaining 1 tablespoon oil and sprinkle over the casserole. Bake until the sauce is bubbling and the topping is golden brown, 20 to 25 minutes.
Make Ahead Tip: Prepare through Step 4; refrigerate for up to 1 day. Let stand at room temperature for 30 minutes and finish Step 5 before baking.
For sustainable chunk light tuna, look for the blue Certified Sustainable Seafood label from the Marine Stewardship Council. The certification means the tuna was troll- or pole-and-line caught, which are sustainable for the fish and the environment. We call for chunk light tuna because it's significantly lower in mercury than albacore (“solid white” tuna)—making it a better choice for health, especially for pregnant women, nursing mothers and children. According to the FDA and EPA, these at-risk groups should still limit their consumption of lower-mercury tuna to 12 ounces a week. The Environmental Working Group (EWG) suggests limiting it even more. (For more info, see the EWG's Seafood Calculator at ewg.org.)
Don't toss your stale bread! Make your own breadcrumbs. Trim off any tough crusts and tear or cut the bread into pieces. Process in a food processor until coarse or fine crumbs form. Use right away or freeze in an airtight container for up to 6 months. Thaw before using. One slice of bread makes about ½ cup fresh coarse breadcrumbs.