Bring a large pot of salted water to a boil. Add the elbow macaroni and cook according to package directions until al dente. Drain well and set aside.

While the pasta cooks, heat olive oil in a large skillet over medium-high heat. Add ground beef and diced onion. Cook, breaking up the beef with a spoon, until browned and no pink remains, about 6-8 minutes. Drain any excess grease. Stir in the taco seasoning mix and water. Bring to a simmer and cook for 2-3 minutes until thickened. Remove beef from the skillet and set aside.

In the same large skillet (no need to wash), melt butter over medium heat. Whisk in the flour and cook for 1 minute, stirring constantly, to create a roux.

Gradually whisk in the warm milk until smooth and thickened, about 3-5 minutes. Reduce heat to low. Add shredded cheddar cheese and Monterey Jack cheese, stirring until completely melted and smooth. Stir in the sour cream, undrained diced tomatoes with green chilies, chopped cilantro, and diced jalapeño (if using). Season with salt and black pepper to taste.

Add the cooked macaroni and the prepared taco beef to the cheese sauce in the skillet. Stir gently until everything is well combined and heated through.

Serve immediately, garnished with crushed tortilla chips, extra shredded cheese, fresh cilantro, a dollop of sour cream, and salsa if desired.