Sour Cream and Onion Tuna Noodle Casserole

Preheat oven to 375°F. Lightly butter the insides of a large baking dish and set aside.

Heat butter in a large heavy saucepan with tall sides over medium to medium-high heat. Add onion and celery and cook until just softened, about 5 to 7 minutes. Stir in garlic and cook for 1 minute more. Sprinkle flour over the top and stir lightly for 1 minute. Slowly add milk and half-and-half, and then simmer until mixture is lightly bubbling and thickened, stirring occasionally for about 5 to 7 minutes. Stir in Parmesan and dry mustard, and then season to taste with salt and pepper. Remove saucepan from heat and stir in peas.

While sauce is cooking in the saucepan, cook noodles in a large pot of boiling salted water until just al dente. Drain noodles, reserving 1/2 cup of noodle cooking liquid. Transfer noodles to a very large mixing bowl. Pour warm sauce over noodles. Then add sour cream, Gruyere, dill, and chives, and fold to blend. Add reserved noodle cooking liquid a bit at a time until mixture is creamy and moistened to your liking (you may not want to use the full 1/2 cup of liquid). Fold in tuna, taking care to leave some chunks intact.

Transfer mixture to prepared baking dish. Sprinkle with crushed potato chips. Tent dish loosely with foil and bake for 20 to 30 minutes, or until hot throughout. Remove foil and bake uncovered for another 5 to 10 minutes, or until top is golden brown and the filling bubbles. Remove from oven, sprinkle with a bit more fresh chopped dill and chives, and serve immediately.