Combine roast, flour, salt and pepper in a large ziplock bag. Seal and shake to coat beef.: As you shake the bag, you should hear the dry flour whisper against the meat and see the cubes pick up a thin, even coating. This coating is important because it forms the base for browning, which creates a fragrant crust that enhances the final broth. If you skip this stage, the soup will lose some of its body and the visual appeal of golden edges. A common error is adding too much flour so the pieces clump together, which prevents even searing; use a light dusting and shake gently.
Heat oil in a large skillet over medium-high heat. Remove beef from ziplock bag, discard any excess flour mixture. Sauté beef until browned, about 5 minutes.: When the oil shimmers and moves easily across the pan, it is hot enough. Add the coated beef in batches so the pieces sizzle on contact, creating a rich, browned surface. The sound of a steady sizzle and the scent of caramelizing meat are good indicators you are developing flavor. If the pan is overcrowded, the meat will steam and not brown, so work in small batches to maintain that satisfying sear.
Place meat into slow cooker. Add onion soup mix, beef broth, tomato paste and Worcestershire sauce. Cook on LOW for 8 hours, until beef is tender.: After searing the meat, transfer it to the slow cooker and leave behind any loose flour. The seared exterior should be a deep golden to brown color, offering both visual and aromatic signals of proper caramelization. Keeping excess raw flour can thicken the broth unevenly and create pasty bits, so it is best to discard it before the beef goes into the cooker.
Add noodles to slow cooker. Cover and cook on LOW for 15-30 minutes, until noodles are tender.: Listen for an even, steady sizzle and watch for a rich Maillard color on the edges. That five minute window is a guideline, but focus on color and aroma; you want savory, complex notes, not a gray, steamed appearance. Underbrowned meat leaves the broth less flavorful, while burned bits will make it bitter, so adjust the heat if the pan is cooking too aggressively.
Place meat into slow cooker: When the browned cubes hit the slow cooker, they bring along all the fond and aroma that will steep into the broth. Spread them evenly so the liquid can surround each piece, promoting even tenderizing. A mistake here is stacking too much meat in one spot, which can slow the exchange of heat and lengthen the tenderizing process.
Add onion soup mix, beef broth, tomato paste and Worcestershire sauce: Swirl these flavor builders into the cooker so they dissolve and mingle. The broth will darken and the tomato paste will infuse a faint, balancing tang while the Worcestershire deepens umami. Stir gently to distribute everything; clumps of paste or mix can create pockets of uneven flavor. If the mixture tastes flat, check the seasoning after cooking and adjust rather than oversalting at the start.
Cook on LOW for 8 hours, until beef is tender: During this long simmer the meat fibers relax and the connective tissue softens into tender, yielding bites. The aroma will evolve from meaty to richly stewed as collagen converts to gelatin, giving body to the broth. Resist the urge to open the lid frequently, because that releases heat and extends cooking time. If the meat is tough at the end, continue cooking rather than increasing temperature abruptly, which can toughen fibers.
Add noodles to slow cooker: When you drop the dry egg noodles into the simmering soup, they will start absorbing liquid immediately. Stir them gently into the broth so they are submerged and cook evenly. The noodles will swell and soften, taking on the savory broth flavors, which is what gives the soup its hearty character. Adding them too early will make them mushy and sap too much of the liquid.
Cover and cook on LOW for 15 to 30 minutes, until noodles are tender: Watch for the noodles to go from opaque to glossy and tender, and smell the warm, wheaty fragrance they release as they finish. The lower end of the window gives slightly firmer noodles, while the longer end yields softer, melt in your mouth strands. Check at intervals to find your preferred texture; overcooked egg noodles will break down and thicken the soup excessively. If the broth becomes too thick, a splash of extra hot broth can loosen the consistency.