Preheat the oven to 400 degrees F. Bring a large pot of salted water to a boil. Add the noodles and cook according to the package directions. Reserve 1/2 cup of the cooking water, then drain the noodles and return to the pot.
Meanwhile, melt 1 tablespoon of the butter in a large skillet over medium-high heat. Add the scallion whites and sliced celery. Cook, stirring occasionally, until softened, about 2 minutes. Add the evaporated milk, 1/2 teaspoon salt and a few grinds of pepper. Stir in the peas and carrots and the mustard. Bring to a boil, then reduce the heat to medium low. Cook, stirring occasionally, until slightly thickened, 2 to 3 minutes.
Pour the sauce over the noodles; stir in the sour cream, 1/2 cup of the cheese, the tuna and reserved cooking water. Transfer to a 1 1/2-quart baking dish.
Melt the remaining 1 tablespoon butter. Toss with the panko, scallion greens, celery leaves and remaining 1/4 cup cheese; season with salt and pepper. Sprinkle over the casserole. Bake until golden, 15 to 20 minutes.