Place a large skillet or Dutch oven over medium-high heat and brown the ground beef until no pink remains.

Drain any excess rendered fat from the skillet.

Add the diced onion to the beef and sauté for 3 to 4 minutes until translucent.

Stir in the minced garlic and tomato paste, cooking for 60 seconds until the paste darkens slightly.

Add the taco seasoning, beef broth, salsa, and milk to the skillet, stirring to deglaze the bottom.

Incorporate the dry pasta shells and bring the mixture to a rolling boil.

Reduce heat to low, cover the skillet, and simmer for 12 to 14 minutes or until the pasta is al dente.

Remove the skillet from the heat source and stir in the cheddar and Monterey Jack cheeses until completely melted and emulsified into a sauce.

Garnish with chopped cilantro and serve immediately.