Cook egg noodles according to package directions; drain and return to pan to keep warm.
In a large frying pan over medium heat, melt the butter; add the shallots and sauté until translucent.
Increase the heat, being careful to just heat the butter and not burn the shallots. Add the beef tenderloin pieces, season with salt and pepper, and sauté, stirring constantly, until the meat is medium-rare (just pink in the center). Transfer onto a warm platter; cover loosely with aluminum foil to keep warm.
Increase the heat to high, add the beef stock and deglaze the pan (using a spoon or spatula, stirring constantly, deglaze the pan by stirring in all the browned bits from the bottom of the pot). Continue stirring to reduce the stock by about half.
Add the crème fraiche and let simmer, stirring constantly, until the sauce is thickened slightly. Lower the heat to a very low simmer and whisk in the sour cream and Dijon mustard. Return the meat and all accumulated juices to the pan. Keeping the heat very low, let simmer just until meat is warmed through. Season to taste with additional salt and pepper.
Remove from heat and serve over cooked noodles, if desired.
Makes 6 to 8 servings.
* Check out Karen's recipe for easy-to-make homemade Beef Stock. If you don't have homemade beef stock, you may substitute a good-quality store-bought broth.
Beef Stroganoff Recipe: https://whatscookingamerica.net/karencalanchini/beefstroganoff.htm