Bring a large pot of water to boil for the pasta. Salt the water and cook the pasta to less than al dente, 5 to 6 minutes. Drain and drizzle with a touch of oil to keep the pasta loose.
In a large bowl, flake the tuna and add the herbs (reserving a tablespoon or 2 for breadcrumbs), onions, mustard, celery, lemon juice, EVOO and some salt and pepper. Add the pasta and combine well. Transfer to a baking dish or large casserole dish.
In a medium saucepot, melt 4 tablespoons of butter. Whisk in the flour and cook 1 minute. Then whisk in the milk and season with salt and pepper, thicken to coat the back of a spoon. Stir in the Cheddar in a figure-eight motion. Pour the sauce evenly over the pasta and tuna. Gently combine with a spoon to settle the sauce. Cool and store for a make-ahead meal.
Melt the remaining 4 tablespoons of butter in a small pan and add the breadcrumbs and reserved herbs, toss to evenly distribute the butter, remove from heat and transfer to airtight container.
To prepare, preheat the oven to 400 degrees F and bring the casserole to room temperature. Top the casserole with breadcrumbs and Parm cheese. Bake until golden and hot through.