Preheat oven to 350°F / 177°C. Spray a 9x13 baking dish (or any casserole dish of similar size) with nonstick cooking spray or brush with some oil.
Boil noodles according to package directions.
While pasta boils, heat a saucepan over medium heat. Add butter and cook until butter is melted. To melted butter, add flour and celery seeds and stir until no dry spots remain. Continue cooking until butter-flour mixture starts turns golden brown, 2 to 3 minutes.
Slowly pour in milk while whisking. Bring the mixture to a simmer (adjust heat as needed), whisking constantly. Simmer until mixture thickens slightly, about 3 minutes more. Remove from heat and whisk in cheese.
Gently fold cooked pasta, peas, tuna, and lemon zest into sauce. Taste and season with some salt and black pepper (don’t skip this step - the dish will be slightly salty from the cheese, but will likely need more salt).
Spread mixture out in prepared baking dish.
Gently stir together melted butter and cornflakes. Spread cornflakes over top of the casserole.
Bake until the topping is golden brown and the filling is heated all the way through, 20 to 25 minutes.