Spray an 8-inch square baking dish with cooking spray and preheat the oven to 375°F.

Melt the unsalted butter in a heavy large saucepan over medium heat.  Add the leeks and celery seed to the pan and season lightly with kosher salt.  Cover the pan and cook the leeks, stirring occasionally, until they are tender but not browned, about 8 minutes.  Add the flour; stir for one minute.  Gradually add the milk and half and half, and simmer, stirring frequently, until slightly thickened, about 5 minutes.  Stir in the lemon juice, season to taste with salt and pepper, and remove from the heat.

Meanwhile, heat a large pot of boiling water, salt the water and cook the egg noodles until they are almost tender—they should still have a slight bite to them as they will cook more in the oven.  Reserve ¾ cup of the pasta cooking water and drain the noodles.  Transfer the noodles to a large bowl, add the leek sauce, gruyère cheese, and the dill and stir gently to combine, adding some of the reserved pasta water a tablespoon at a time until the mixture is moist and creamy (you will need about 8 tablespoons.)  Gently fold in the tuna and transfer to the prepared baking dish. Cover with foil until heated through, about 20 minutes, then remove the foil and sprinkle the casserole with the potato chips.  Continue to bake, uncovered, until the top is golden brown and the mixture is bubbling, about 10 more minutes.  Serve immediately.