Preheat your oven to 350°F (175°C).
In a large skillet, brown the ground beef and diced onion over medium heat, breaking up the meat as it cooks.
Once cooked, drain any excess fat from the skillet and return it to the heat.
Stir in cooked macaroni, diced tomatoes with green chilies, ketchup, mustard, garlic powder, paprika, salt, and pepper.
Transfer the mixture into a greased casserole dish and top with shredded cheddar cheese.
Cover the casserole with aluminum foil and bake for 20 minutes.
After 20 minutes, remove the foil and bake for an additional 10 minutes, or until the cheese is bubbly and golden.
Let the casserole sit for a few minutes before serving.
