Preheat oven to 350°F. Prepare pasta according to package directions.
Meanwhile, cook green beans in boiling salted water to cover 30 seconds to 1 minute or until crisp-tender; drain. Plunge into ice water to stop the cooking process; drain.
Melt 2 Tbsp. butter in a large skillet over medium-high heat. Add leeks, and sauté 2 minutes; add mushrooms, and sauté 5 minutes or until lightly browned. Transfer leek mixture to a small bowl. Wipe skillet clean.
Melt remaining 4 Tbsp. butter in skillet over medium heat; whisk in flour, and cook, whisking constantly, 2 minutes. Gradually whisk in cream and broth. Bring mixture to a boil, stirring often. Reduce heat to medium-low; gradually whisk in Cheddar cheese and 4 Tbsp. Parmesan cheese until smooth. Stir in salt and pepper.
Stir cream mixture into pasta. Stir in tuna, chives, tarragon, beans, and leek mixture; transfer to a lightly greased 13- x 9-inch baking dish.
Stir together potato chips, panko, and remaining 2 Tbsp. Parmesan cheese in a small bowl; sprinkle over pasta mixture.
Bake for 35 to 40 minutes or until bubbly. Let stand 5 minutes before serving.