Season short ribs with 1/2 teaspoon salt and pepper. Heat neutral oil in a large skillet over medium-high heat and brown ribs in two batches for 3 minutes per broad side, then transfer to a 6-quart slow cooker.
Soften onion in the skillet for 5 minutes, add garlic for 1 minute and pour in wine, scraping up the browned bits. Transfer everything to the cooker with the 2 cups broth, thyme, Dijon and Worcestershire.
Cover and cook on low for 7 to 8 hours until the meat is fork-tender. Near the end, heat oven to 425°F, toss both mushrooms with olive oil and remaining salt and roast for 20 to 25 minutes until browned.
Lift ribs onto a tray, remove bones and hard fat and pull the meat into large pieces. Strain cooking liquid, keep the soft onions, skim fat thoroughly and measure 4 cups liquid, adding extra broth as needed.
Return measured liquid and onions to the cooker and turn to high. Stir in dry egg noodles, cover and cook for 20 to 30 minutes, stirring after 10 and 20 minutes, until tender.
Fold in beef and roasted mushrooms, cover for 5 minutes and check that the beef reaches 165°F. Switch off, let bubbling settle for 3 minutes and fold in crème fraîche.
Divide among six bowls while the noodles still have shape and the sauce coats them loosely.