Brown the meat (optional but recommended): In a skillet over medium heat, cook the ground beef with the diced onion until the meat is browned and the onion softens, 5–7 minutes.
Drain excess fat. Season lightly with salt and pepper.
Layer the base: Add the cooked beef and onion to the crockpot. Sprinkle in the taco seasoning.
Add the Rotel, black beans, corn, salsa, and condensed cheese soup.
Add liquid: Pour in 1 cup of broth and stir to combine. The mixture should be loose but not soupy. You can add up to an extra 1/2 cup later if it looks dry.
Cook low and slow: Cover and cook on Low for 4–6 hours or High for 2–3 hours, until the flavors are blended and the mixture is bubbling around the edges.
Decide on your starch: If using pasta shells, cook them separately to al dente according to package directions, then drain.
If using rice, have it cooked and ready. Stir in the pasta or spoon the mixture over rice when serving.
Add the cheese: Stir in 1 cup shredded cheese until melted. Taste and adjust seasoning with salt and pepper.
Finish and serve: Top the crockpot with the remaining 1 cup cheese, cover for 5 minutes to melt.
Scoop into bowls and add your favorite toppings like sour cream, cilantro, and crushed tortilla chips.