Cook rotini pasta according to package directions. Preheat oven to 375 F. Grease 9 x 13-inch baking dish.
In a large saucepan over medium heat, melt 3 tablespoons butter with the dried onion. Add flour and stir for 1 to 2 minutes, until flour begins to brown and have a nutty smell. Slowly whisk in milk, then whisk in cream of mushroom soup until sauce thickens. Add ½ cup of the cheddar cheese, stirring until melted.
Stir in rotini, peas, mushrooms, and tuna. Pour into casserole dish and top with remaining cheese. Melt remaining butter and toss with the bread crumbs. Scatter crumbs over cheese.
Bake for 15 to 20 minutes, or until crumb topping is nicely browned.
Per serving: 631 calories, 25 grams fat (13 grams saturated fat), 147 milligrams cholesterol, 666 milligrams sodium, 58 grams total carbohydrates, 1.5 grams fiber, 42 grams protein.
