Step 1

Preheat oven to 375°F. In a large saucepan cook pasta according to package directions, adding diced potatoes during the last 4 minutes of cooking time. Drain and set aside.

Step 2

Meanwhile, for sauce, in a large Dutch oven melt 2 Tbsp. of the butter. Cook onion in butter about 3 minutes, until tender. Stir in flour and a pinch of salt and pepper. Cook and stir 2 minutes more. Whisk in milk all at once. Add lemon peel. Cook and stir until thickened and bubbly. Whisk in provolone cheese until melted. Gently fold pasta, potatoes, tuna, olives, and asparagus into sauce. Pour into a 3 quart rectangular baking dish .

Step 3

For topping, in a small bowl melt the remaining 1 Tbsp. of butter. Stir in bread crumbs and Parmesan. Sprinkle on casserole. Bake, uncovered, for 25 to 30 minutes, until heated through and topping is golden.