Boil the pasta: Bring a large pot of salted water to a boil. Cook the macaroni until just shy of al dente. Drain and set aside, reserving 1/2 cup of pasta water.
Sauté aromatics: In a wide pot or deep skillet, melt 1 tablespoon of butter over medium heat.
Add chopped onion and cook until translucent, 3–4 minutes. Stir in garlic for 30 seconds.
Boost the chili: Add 1 teaspoon chili powder, 1/2 teaspoon cumin, and a small squeeze of tomato paste. Cook for 30–60 seconds to toast the spices.
Add canned chili: Stir in the canned beef chili.
Bring to a gentle simmer. Taste and season with salt and pepper as needed.
Make a quick cheese sauce: In a small saucepan, melt 2 tablespoons of butter over medium heat. Whisk in 2 tablespoons of flour and cook 1 minute.
Slowly whisk in 1 cup of milk until smooth and slightly thickened, 3–5 minutes. Off heat, stir in 1 to 1 1/2 cups shredded cheddar until melted. Season with a pinch of salt and pepper.
Combine: Add the drained pasta to the chili.
Pour in the cheese sauce and stir until everything is evenly coated. If it’s too thick, loosen with a splash of the reserved pasta water or more milk.
Adjust and serve: Add hot sauce if you want more heat. Top with extra cheese, sliced green onions, or a dollop of sour cream.
Serve warm.