Preheat oven to 350 degrees F. Lightly coat a 2-quart rectangular baking dish with cooking spray; set aside. In a covered medium saucepan cook potatoes and orzo in a large amount of boiling salted water about 10 minutes or until tender. Drain and set aside.
In a large bowl combine cooked potatoes, orzo, broth, cream cheese, milk, and thyme. Stir in tomatoes, peas, olives, and onion. Stir in 3/4 cup of the Parmesan cheese. Stir in tuna. Transfer to the prepared baking dish, spreading evenly.
In a small bowl combine panko and the remaining 1/4 cup Parmesan cheese; stir in melted butter. Sprinkle overcasserole. Bake, uncovered, for 25 to 30 minutes or until golden brown and bubbly.