How to Make It
Heat oven to 400° F. Melt 4 tablespoons of the butter in a large saucepan over medium heat. Add the flour and cook, stirring, until foamy, 1 to 2 minutes (do not let the mixture darken). Slowly whisk in the milk. Bring to a simmer and cook, whisking often, until thickened, 3 to 5 minutes. Remove from heat and whisk in the sour cream, 1 teaspoon salt, and ¼ teaspoon pepper.
Meanwhile, cook the noodles according to the package directions; drain and return them to the pot. Pulse the bread in a food processor until coarse crumbs form. Melt the remaining 2 tablespoons of butter in a microwave-safe bowl.
Add the milk mixture, chicken, spinach, and thyme to the noodles and toss to combine. Transfer to a 9-by-13-inch or some other 3-quart baking dish, sprinkle with the bread crumbs, and drizzle with the melted butter.
Bake until the bread crumbs are golden and the filling is bubbling, 8 to 10 minutes. Let cool for 5 minutes before serving.