Brown the meat: In a skillet over medium heat, cook ground beef with diced onion until browned, about 5-7 minutes. Drain excess fat. Add garlic and sauté for 30 seconds until fragrant.
Add to crockpot: Transfer beef mixture to a 6-quart slow cooker. Add taco seasoning, Rotel, tomato sauce, and beef broth. Stir to combine.
Cook the base: Cover and cook on low for 4 hours or high for 2 hours.
Add pasta: Break spaghetti in half and stir into the crockpot, ensuring it’s submerged. Cover and cook on high for 20-30 minutes, stirring halfway, until pasta is al dente.
Finish with cheese: Stir in cream cheese and half the cheddar until melted. Sprinkle remaining cheddar on top, cover, and let sit for 5 minutes to melt.
Serve: Garnish with cilantro, jalapeños, sour cream, or avocado, and serve warm.