Brown the beef: In a large deep skillet over medium-high heat, brown the ground beef with the onion, breaking it up as it cooks, about 6-8 minutes. Drain off the excess grease.
Bloom the seasoning: Add the garlic and taco seasoning directly to the skillet. Stir and cook for about 60 seconds until fragrant.
Add the liquids: Stir in the diced tomatoes with green chilies, tomato sauce, and beef broth. Bring to a simmer.
Cook the pasta in the skillet: Stir in the pasta shells, cover, and simmer for 12-15 minutes, stirring occasionally, until the pasta is tender and most of the liquid is absorbed.
Melt in the cream cheese: Reduce the heat to low and stir in the cubed cream cheese until fully melted and the sauce turns creamy.
Finish with cheese: Remove from the heat and stir in the cheddar and Monterey Jack until melted.
Serve: Top with sliced green onions and serve warm.
Notes
Bloom the taco seasoning in the hot beef fat for about 60 seconds before adding liquid, it makes the flavor taste like more than a packet stirred into noodles. Cook ground beef to an internal temperature of 160°F.