Pat the beef dry and season it with salt and pepper. Melt 1 tablespoon butter in a large skillet over medium-high heat. Add the meat (in batches if necessary) and cook, stirring occasionally (or turning) for 3 minutes, until the meat is browned on all or at least most sides. Transfer to a plate and pour off any remaining fat from the pan.
Return the pan to medium-high heat. Melt 1 more tablespoon of the butter, then add the shallots and cook, stirring often, until they are slightly soft, about 2 minutes. Add the mushrooms and thyme leaves, season with salt and pepper, and sauté for another 7 minutes until tender and lightly browned, and the mushroom juices evaporate. Add the cooked mushrooms to the beef on the plate.
Return the pan once more to medium high heat (don’t clean the pan!) Add the third tablespoon of butter and when it is melted, stir in the flour. Stir for about 2 minutes until the flour turns golden brown. Add the beef broth and bring to a simmer, stirring and scraping the bottom of the pan to release any browned bits. Return the meat and mushrooms, along with any accumulated juices, to the pan. Add the sour cream, mustard and paprika, stir to blend, and remove from the heat.
Meanwhile, add the noodles to the boiling water and cook for 8 or 9 minutes, or according to package directions, until tender. Drain and transfer to a shallow serving bowl. Add the remaining tablespoon of butter and toss until the butter melts and coats the noodles.
Pour the stroganoff mixture over the hot noodles, and sprinkle over the parsley. Serve hot.