1. Cook the macaroni according to the package directions until al dente. Drain, rinse under cold water until cool, and drain again.
2. Meanwhile, heat a large Dutch oven over high heat until smoking hot. Add the beef, 1 teaspoon salt and ¼ teaspoon black pepper and spread in a single layer, breaking the meat into small bits. Cook until the bottom is nicely browned, about 5 minutes. Stir well, then push the beef to one side of the Dutch oven.
3. Add the onion, garlic, ½ tsp salt and ¼ tsp black pepper to the other side. Cook, stirring occasionally, until the onion is translucent, about 3 minutes. Stir the onion mixture into the beef and push to one side of the Dutch oven again.
4. Add the tomato paste to the other side, reduce the heat to medium, and stir for 1 minute. Stir into the beef until well mixed. Add the mustard powder, paprika, and cayenne and stir for 1 minute.
5. Reduce heat to medium-low. Push everything to one side of the pot. Add the butter to the other side. When it melts, dump the flour over the butter. Stir the flour and butter until the raw flour cooks out, about 2 minutes. Stir the roux into the meat mixture.
6. While stirring, add the milk ¼ cup at a time. Keep stirring until the mixture bubbles steadily for 2 minutes. Reduce the heat to low and stir in the cheese until melted.
7. Add the pasta and fold until evenly coated and hot. Season with salt and pepper and serve immediately.