1 Heat the oven to 400F.
2 Cook the pasta: Bring a large pot of salted water to a boil, and cook the pasta until al dente. Drain, rinse with cold water, and set aside.
3 Make the chili: Heat the oil in a large (12-inch) oven-safe skillet over medium heat. Add the onions and cook until softened, 2 to 3 minutes. Add the tomatoes and cook until they have broken down slightly and some of the liquid has evaporated, 3 to 5 minutes more.
Add the ground beef, black pepper, 1 tablespoon chili powder, and 1 tablespoon salt to the onion-tomato mixture. Cook, breaking up the ground beef with a wooden spoon, until brown and fully cooked, 5 to 8 minutes. Remove from heat.
4 Make the cheese sauce: Melt 2 tablespoons of butter in a saucepan over medium heat. Add the flour and cook, stirring occasionally, until bubbly and slightly golden, 1 to 2 minutes.
Slowly whisk the milk into the butter-flour mixture. At first, the mixture will seize up and look crumbly, but will smooth out as you keep adding milk and whisking.
Once all the milk is added, bring to a simmer. Whisk frequently and scrape along the bottom of the pan, so it doesn’t burn.
Cook until the sauce has thickened slightly and seems creamy, 10 to 15 minutes. Remove from the heat, and add the shredded cheese and mustard. Stir until the cheese has melted. Taste and add salt and pepper as needed.
5 Assemble the mac and cheese: Combine the cooked pasta, chili mixture, and cheese sauce – you can do this in your skillet if it’s big enough, otherwise use a large bowl. Mix until evenly combine, then transfer back to the skillet used to cook the chili. (Or transfer to a baking dish if your skillet isn’t big enough.)
6 Top with toasted breadcrumbs: Melt the 2 tablespoons of butter and mix with the breadcrumbs, 1/2 teaspoon chili powder, and 1/4 teaspoon salt. Sprinkle evenly over top the chili mac and cheese.
7 Bake the mac and cheese: Place the chili mac and cheese in the oven and bake until golden on top and the cheese sauce is bubbling, about 20 minutes.
Leftovers can be kept refrigerated for about a week. Reheat individual portions in the microwave, or larger amounts in a low oven. Leftovers can also be frozen in individual portions for up to 3 months; reheat in the microwave or in a low oven.