Directions

1. Position oven rack 8 inches from heat source; preheat broiler. Lightly coat a 9-by-13-inch broiler-safe baking dish with cooking spray. Bring a large pot of water to a boil over medium-high heat. Cook pasta, stirring occasionally, until al dente, about 9 minutes. Reserve 1 cup cooking water; drain the pasta well and transfer to a large bowl.
2. Heat oil in a medium saucepan over medium-high heat. Add shallot; cook, stirring often, until softened, about 2 minutes. Stir in flour; cook, stirring constantly, 1 minute. Gradually whisk in milk; cook, whisking often, until slightly thickened, about 2 minutes. Whisk in 1/4 cup Parmesan until melted. Remove from heat; stir in pesto, salt, pepper and 3/4 cup reserved cooking water.
3. Pour the sauce over the pasta; stir, adding some or all of the remaining 1/4 cup of reserved cooking water until desired consistency is reached. Gently fold in tuna, tomatoes and spinach until evenly distributed. Spoon into the prepared baking dish. Sprinkle with panko and the remaining 1/4 cup Parmesan. Broil until the casserole is heated through and the top is golden brown, about 10 minutes.