Boil the egg noodles in salted water until tender, drain them and keep them warm.
Season the beef with 1 teaspoon salt and the pepper, then sear it in 2 batches with 1 tablespoon oil over high heat for 90 seconds per side until at least 145°F; transfer it to a plate.
Add the remaining oil and cook the mushrooms and onion for 7 minutes until browned, then stir in the garlic for 30 seconds.
Sprinkle in the flour, stir for 1 minute and whisk in the broth, mustard and remaining salt; simmer for 4 minutes.
Remove the skillet from direct heat, stir in the sour cream and beef and serve immediately over the noodles.