Preheat oven to 425 degrees F.
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 thickened 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.