Step 1

Preheat the oven to 350 degrees F.

Step 2

Melt 3 tablespoons butter in a large skillet over medium-high heat. Add the onions and sprinkle with 1 teaspoon salt and 1/2 teaspoon pepper and cook, stirring occasionally, until the onions are very soft and just begin to brown, about 20 minutes. Add 1 tablespoon thyme and cook for an additional minute. Set aside.

Step 3

Meanwhile, bring a large pot of salted water to a boil. Cook the noodles according to the package directions. Reserve 1/2 cup of the pasta water. Drain the noodles and return them to the pot.

Step 4

While the noodles are still warm, add the cooked onions, sour cream, cream cheese, reserved pasta water and 1/2 cup Gruyere. Stir until the sauce is smooth and creamy and coats the noodles. Fold in the chicken. Season with salt and pepper. Transfer to a 2-quart baking dish.

Step 5

Microwave the remaining 5 tablespoons butter in a medium microwave-safe bowl until melted. Add the fried onions, breadcrumbs, remaining 1/2 cup Gruyere and 1 tablespoon thyme to the butter and toss until the breadcrumbs are coated. Sprinkle the buttered breadcrumb mixture evenly over the noodles to cover completely.

Step 6

Bake until the topping is golden brown, 35 to 40 minutes.