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1
Preheat oven to 375 degrees F. Lightly butter the insides of a large baking dish and set aside.
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2
Heat butter in a large heavy saucepan with tall sides over medium to medium-high heat.
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3
Add onion and celery and cook until just softened, about 5 to 7 minutes.
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4
Stir in garlic and cook for 1 minute more.
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5
Sprinkle flour over the top and stir lightly for 1 minute.
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6
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.
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7
Stir in Parmesan and dry mustard, and then season to taste with salt and pepper.
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8
Remove saucepan from heat and stir in peas.
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9
While sauce is cooking in the saucepan, cook noodles in a large pot of boiling salted water until just al dente.
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10
Drain noodles, reserving 1/2 cup of noodle cooking liquid.
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11
Transfer noodles to a very large mixing bowl.
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12
Pour warm sauce over noodles.
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13
Then add sour cream, Gruyere, dill, and chives, and fold to blend.
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14
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).
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15
Fold in tuna, taking care to leave some chunks intact.
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16
Transfer mixture to prepared baking dish.
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17
Sprinkle with crushed potato chips.
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18
Tent dish loosely with foil and bake for 20 to 30 minutes, or until hot throughout.
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19
Remove foil and bake uncovered for another 5 to 10 minutes, or until top is golden brown and the filling bubbles.
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20
Remove from oven, sprinkle with a bit more fresh chopped dill and chives, and serve immediately.
