Cook the macaroni according to the package directions. Drain and set aside.
In a large saucepan, cook the ground beef and chopped onion over medium heat until the meat is fully browned, breaking it into crumbles as it cooks. This should take about 6-8 minutes.
Add the minced garlic and sauté for an additional minute to release its aroma.
Drain any excess fat from the meat mixture.
Mix in the diced tomatoes, kidney beans, tomato paste, green chiles, salt, chili powder, cumin, and pepper. Stir well to combine.
Bring the mixture to a boil, then reduce the heat and simmer uncovered for 10 minutes to let the flavors meld.
Preheat your oven to 375°F (190°C).
Incorporate the cooked macaroni into the beef mixture, stirring until evenly coated.
Transfer the mixture into two greased 2-quart baking dishes.
Sprinkle Monterey Jack cheese evenly over the top of each casserole and layer the jalapeno slices on top.
Cover the dishes with foil and bake for 30 minutes.
Uncover and bake for an additional 10 minutes, until the cheese is bubbly and golden.
Enjoy one casserole fresh, and let the second cool completely before covering it tightly and freezing for up to 3 months.