Preheat the oven
Preheat your oven to 400°F. This ensures it’s ready for baking as you finish assembling the dish.
Cook the pasta
Bring a large pot of salted water to a boil. Add the elbow pasta and cook until al dente. Drain, rinse with cold water to stop cooking, and set aside.
Make the chili
Heat vegetable oil in a large oven-safe skillet over medium heat. Add the diced onion and cook for 2–3 minutes until softened. Stir in the diced tomatoes and cook until they break down slightly, about 3–5 minutes.
Add the ground beef, black pepper, chili powder, and salt. Cook, breaking up the meat with a spoon, until browned and fully cooked, about 5–8 minutes. Remove from heat.
Prepare the cheese sauce
In a medium saucepan, melt 2 tablespoons of butter over medium heat. Add the flour and stir until bubbly and golden, about 1–2 minutes. Gradually whisk in the milk, ensuring a smooth consistency.
Simmer the sauce for 10–15 minutes, whisking frequently, until thickened. Remove from heat and stir in the shredded cheese and mustard until fully melted. Adjust salt and pepper to taste.
Combine the components
In the skillet with the chili, add the cooked pasta and cheese sauce. Stir until evenly combined. If your skillet isn’t large enough, transfer everything to a large mixing bowl before combining, then return it to the skillet or a baking dish.
Add the breadcrumb topping
Melt 2 tablespoons of butter and mix with breadcrumbs, chili powder, and salt. Sprinkle the breadcrumb mixture evenly over the top of the chili mac and cheese.
Bake
Place the skillet in the preheated oven and bake for 20 minutes, or until the top is golden brown and the cheese sauce is bubbling.
Serve
Let the dish cool for 5 minutes before serving. Enjoy warm with optional toppings like sour cream, chopped green onions, or diced jalapeños.