Brown the ground beef with the diced onion in a large pot or deep skillet over medium-high heat, breaking it up, about 7 minutes. Drain off excess fat.
Add the garlic, chili powder, cumin, smoked paprika, and salt. Cook for 1 minute until fragrant — this blooms the spices and deepens their flavor.
Pour in the diced tomatoes with their juice and the beef broth. Bring to a boil, scraping up any browned bits from the bottom.
Stir in the dry pasta. Reduce to a simmer, cover, and cook 12 to 15 minutes, stirring every few minutes, until the pasta is tender and most of the liquid has been absorbed.
Remove the pot from the heat completely. Add the shredded cheddar a handful at a time, stirring until melted and smooth.
Let it sit for 5 minutes to thicken, then top with sliced green onions and serve.