Cook the pasta: Bring a large pot of salted water to a boil. Add the pasta and cook according to the package instructions until al dente. Drain and set aside.

1. Cook the ground beef: While the pasta is cooking, heat olive oil in a large skillet over medium-high heat. Add the ground beef and cook, breaking it apart with a spoon, until browned and fully cooked through (about 6-8 minutes). Drain any excess fat.
2. Sauté the onion and garlic: Add the diced onion to the skillet with the cooked beef and cook for 3-4 minutes until softened. Add the minced garlic and cook for another 30 seconds, until fragrant.
3. Add the taco seasoning: Stir in the taco seasoning and diced tomatoes. Add salt and pepper to taste. Cook for 3-4 minutes, stirring occasionally, until the mixture is well combined and heated through.
4. Combine the pasta and sauce: Stir the cooked pasta into the beef mixture, combining it thoroughly. Add the milk and sour cream (if using), stirring until everything is coated in a creamy sauce.
5. Add the cheese: Sprinkle the shredded cheddar cheese over the top and stir until the cheese is melted and everything is creamy and cheesy. If the sauce is too thick, add a little extra milk to loosen it up.
6. Serve: Top with your favorite taco toppings, such as sliced jalapeños, chopped cilantro, diced tomatoes, shredded lettuce, or crushed tortilla chips. Serve immediately.