Start by grating the cheddar and Monterey Jack cheese from a block and set them aside. Letting the cheese reach room temperature before adding it to the sauce ensures smooth melting.

In a large, high-walled pot or Dutch oven, cook the ground beef over medium-high heat, breaking it apart as it browns. Once fully cooked, drain any excess grease and set the beef aside.

Using the same pot, melt the butter over medium heat. Add the minced garlic and sauté for about one minute, stirring frequently to prevent burning.

Return the ground beef to the pot and add the taco seasoning, Worcestershire sauce, tomato paste, beef broth, chicken broth, whole milk, and Rotel tomatoes with their juices. Stir well to fully combine the ingredients and create a rich, flavorful base.

Bring the mixture to a gentle boil, then add the pasta. Submerge the shells in the liquid, ensuring they are evenly distributed. Cover the pot and allow the pasta to cook according to the package instructions. About halfway through the cooking process, use a silicone spatula to gently stir the bottom of the pot, preventing any ingredients from sticking or scorching.

Once the pasta is tender, remove the lid and test a noodle for doneness. If it requires more cooking time, cover and allow it to simmer for a few additional minutes until the desired texture is achieved.

Reduce the heat to low and slowly incorporate the cheeses, stirring continuously until melted and fully combined with the sauce. The dish will appear saucy at first, but as it stands, the pasta will absorb some of the liquid, creating a thicker, creamier consistency.

Serve immediately once the sauce has reached your preferred thickness.