Step 1

Preheat oven to 350°. Cook noodles according to package directions; drain. Transfer to a shallow 3-qt. or 13x9-in. baking dish coated with cooking spray.

Step 2

Meanwhile, in a large nonstick skillet, heat butter over medium-high heat. Add onion; cook and stir until tender. In a small bowl, whisk cornstarch, milk and seasonings until smooth; stir into pan. Stir in broth. Bring to a boil; cook and stir until thickened, about 2 minutes. Stir in 3/4 cup cheese until melted; stir in broccoli and tuna.

Step 3

Spoon over noodles; mix well. Sprinkle with remaining cheese. Toss bread crumbs with melted butter; sprinkle over casserole. Bake, covered, 45 minutes. Uncover and bake until cheese is melted, 15-20 minutes longer.