Season beef pieces with salt and pepper, then dredge in flour. Heat a large skillet (cast iron works super well here - or heat your cast aluminum slow cooker insert if you have one) over MED HIGH heat with olive oil. Shake off excess flour and brown beef in batches (don't overcrowd the pan) for several minutes, until you get a golden brown crust on all sides. Remove beef to plate, and continue with all batches.
When all beef is browned, add beef broth to skillet and use a wooden spoon to gently loosen the browned bits from the bottom of the skillet.
To bottom of a 6 quart slow cooker, add sliced onions, minced garlic, beef base, beer, and mustard. Stir together, then pour in beef broth from skillet. Top that with browned beef, thyme sprigs and bay leaves.
Cover slow cooker and cook on LOW for 8 hours.
About 20-30 minutes before serving, bring a big pot of water to a boil for the egg noodles. Mix 2 Tbsp flour and 3-4 Tbsp beef broth until no lumps remain. Stir into beef stew in slow cooker. Increase heat to HIGH and cook uncovered for 10-15 minutes.
Add egg noodles to boiling water and cook according to package directions. Drain and toss with a bit of olive oil or butter to prevent sticking, and a pinch of minced parsley.
To serve, add some egg noodles to bowl or plate. Top with a few ladles of beef carbonnade and sprinkle with some more minced parsley. Alternatively, add egg noodles to slow cooker and stir it all together, then ladle the egg noodle/beef stew mixture into bowls or onto plates.