This is a surprisingly lowfat version of the classic tuna casserole you grew up with. You make your own creamy mixture, which replaces the high-fat condensed soup in the original version. Fresh bread crumbs toast right on top of the casserole!
Place bread in food processor and pulse for 30 seconds into bread crumbs (makes about 2 1/2 to 3 cups crumbs).
Heat oil in a large skillet over medium heat. Add onions and cook, stirring, until onions are soft and translucent, 8 minutes. Add celery and cook, stirring, until just tender, 6 minutes. Add mushrooms and cook, stirring, until mushrooms release their water, 5 to 7 minutes. Add flour all at once and stir immediately and vigorously with a wooden spoon until flour is completely incorporated into vegetable mixture until there are no flour lumps. Add milk and broth, stir to combine and bring mixture to a boil, stirring frequently. Reduce heat to a vigorous simmer and cook, stirring, until liquid has thicke ned and reduced by about 1/2 cup, 7 to 8 minutes. Add salt and pepper and stir to combine. Combine cooked pasta, vegetable-liquid mixture, broccoli, peas and tuna and toss to incorporate. Pour into a 9 by 13-inch casserole. Top with bread crumbs. Bake for 25 minutes, or until crumbs are golden brown and toasted.