Combine the beef broth and dried porcini in a small saucepan.
Bring to a boil, remove from heat, cover, and let steep for 30 minutes.
With a slotted spoon, lift the porcini out of the broth, chop coarsely and set aside.
Strain the broth through a sieve and set aside.
Heat 1 tbsp of the oil and the butter in a large skillet (not a non-stick) over medium high heat.
When the oil is hot, add the cremini mushrooms and saute, stirring often until they are browned, 5- 6 minutes.
Transfer to a bowl.
Season the beef strips generously with kosher salt and pepper and dredge in the flour.
Add 2 tbsp oil to the pan and heat over medium-high heat.
When oil is hot, add the beef, spreading it in a single layer.
After about a minute, turn each piece to brown the other side.
Don't overcook!
Work quickly, the whole process only takes a couple of minutes.
Remove beef to the same bowl with the cremini.
(Work in smaller batches, if you prefer).
Melt the remaining 3 tbsp butter in the pan over medium-high heat.
Add the onion.
Saute, scraping up the browned meat bits at the bottom of the pan.
When the onions just start to brown (about 4- 5 min), pour in the reserved porcini soaking broth.
Stir in the mustard, Worcestershire sauce, porcini, cremini and beef, along with any accumulated juices.
Bring to a simmer while stirring.
Cook just until the sauce thickens slightly, 1- 2 minutes.
Reduce heat to low and stir in the creme fraiche, just until it's incorporated and heated through.
Taste, and adjust seasonings.
Serve over cooked noodles, sprinkled with parsley.
