Combine roast, flour, salt, and pepper in a large ziplock bag. Seal and shake to coat beef.: Right after sealing the bag, you'll notice the flour clinging to the meat and the aroma of the pepper waking up, which sets the stage for browning. The light coating helps create a thin crust during the sauté, leading to deeper color and flavor as those browned bits dissolve into the cooking liquid. A common slip here is overcrowding the bag, which prevents even coating, so work in a single layer or two batches to ensure every piece gets covered.
Heat oil in a large skillet over medium-high heat. Remove beef from the ziplock bag, discard any excess flour mixture. Sauté beef until browned, about 5 minutes.: When the oil is hot you should hear a steady sizzle as the beef hits the pan, and small brown bits will form on the surface producing a toasty, meaty scent. Browning not only adds flavor but also gives texture contrast in the finished soup. Avoid moving the beef too frequently, because that prevents proper crust formation; if you notice steam instead of sizzle, the pan is too crowded or not hot enough.
Place meat into the slow cooker. Add corn, black beans, pinto beans, diced tomatoes and green chiles, Ranch mix, taco seasoning, beef broth, and enchilada sauce.: As you transfer the browned pieces, you'll see a faint oil sheen and browned fond in the skillet that you can deglaze with a splash of the beef broth to capture those flavors and pour them into the slow cooker. The mixture of beans, corn , and sauces creates a layered broth that will become rich and cohesive over time. A typical pitfall is dumping everything cold into a large, unheated slow cooker; using hot broth or at least prewarming the insert shortens the time it takes to reach a safe simmer.
Cook on LOW for 8 hours or HIGH for 4 hours, or until beef is tender.: During the long, slow cook you will start to smell the spices mellow and meld, and the meat fibers will soften until they practically fall apart when prodded with a fork. This gentle simmer allows connective tissue to break down, resulting in silky, fork tender steak . Be careful not to lift the lid frequently, as every peek lets heat escape and can add an hour to the cooking time. If the meat feels firm at the shorter time, give it another hour and check again rather than upping the heat, which can dry the beef.