Preheat the oven to 375 degrees F.
Using the extra butter, grease a 9x13-inch casserole dish.
In a large, thick-bottomed pot over medium heat, melt 1/2 stick of the remaining butter.
Add the shallots and the celery to the melted butter and cook, while stirring, until softened, about 5 minutes.
Add the flour to the shallot mixture and reduce the heat to low.
Cook the shallot mixture, while stirring, until combined and the flour is golden, about 3 minutes.
In a large pot of salted, boiling water, add the egg noodles and cook until just shy of al dente, about 2 minutes less than the time on the package directions. Drain the noodles, rinse them under cold water, and set them aside.
Add the dry vermouth to the shallot mixture and cook, while stirring, until it begins to simmer, about 1 minute.
Add the milk, the cream, and the stock to the vermouth mixture and stir to combine.
Add the tarragon to the sauce mixture.
Raise the heat to medium-low.
Bring the sauce mixture to a simmer.
Simmer the sauce, while stirring frequently, until the liquid has reduced slightly, about 5-8 minutes.
Add the Gruyère cheese and the mustard to the sauce and stir to combine until the cheese has melted.
Add the turkey to the sauce and stir to combine.
Season the turkey mixture with the salt and the black pepper.
Add the egg noodles to the turkey mixture and stir to combine.
Add the lemon juice to the turkey mixture and stir to combine.
Transfer the turkey mixture to the prepared casserole dish, spreading it out evenly.
Sprinkle the top of the turkey mixture evenly with the panko.
Drizzle the remaining 1 tablespoon of the melted butter evenly over the top of the breadcrumbs.
Bake on the middle oven rack until it is bubbling and the top is golden, about 30-40 minutes.
Serve garnished with the parsley.