Heat multipurpose cooker to the "Sear" setting. (If using a slow cooker, heat large sauté pan on medium high heat.)
Add butter or olive oil to multipurpose cooker (or pan), followed by the onions, mushrooms, and garlic. Sauté for 5 to 7 minutes until onions and mushrooms soften.
In the multipurpose cooker, add lamb meat and bones, water, salt, thyme, and black pepper. Stir until everything is combined, making sure that the bones are submerged in the liquid. Change the multipurpose cooker setting from the "Sear" setting to the "Slow Cook" setting.
(If using a slow cooker, add everything except for the lamb meat and bones to a sauté pan and stir to combine; then transfer everything to a slow cooker. Add the lamb meat and bones and stir well, making sure the bones are submerged in the liquid.)
Slow cook on low for 6 hours on low or for 3 hours on high.
Open the multipurpose cooker (or slow cooker). Remove the lamb bones and set them aside.
Fill a 5-quart pot with water and cook the egg noodles according to the package directions.
Turn on the “Boil” function on the multipurpose cooker. (If you don’t have a “Boil” function, use the “Sear” function.) Add the Dijon mustard and light sour cream to the multipurpose cooker. Stir to combine and wait until it just starts to boil.
(If using a slow cooker, pour 2 cups of the sauce from the slow cooker into a 1.5-quart or larger pot and bring to a light boil.)
In a small bowl, mix the cornstarch and 1 tsp water together then pour into the multipurpose cooker (or pot with sauce if you used a slow cooker). Stir the contents to combine and let boil for 1 to 3 minutes to thicken to your desired preference.
Take any meat that falls off the lamb bones and return it to the multipurpose cooker. (Return sauce from the small pot, if using a slow cooker.)
Add the cooked egg noodles to the slow cooker. Stir until the noodles are coated in the sauce.