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Add olive oil, chopped onion, ground beef, and seasonings to a large pot and saute until the meat is fully cooked and the onions are translucent. Thoroughly mix in tomato paste, saute for 2 minutes, then transfer the meat mixture to a bowl and set aside.
Use the same pot to cook pasta in boiling salted water until al dente, about 2 minutes less than package instructions. While pasta is cooking, warm the milk in a separate pot.
Drain pasta once cooked, ***reserving at least 1 cup of the pasta water.***
Return the empty pot to the stove over medium heat and add butter. When butter melts and foams, add flour and whisk until darkened slightly, 1 to 2 minutes. Add mustard powder and whisk to combine.
Slowly whisk in the warm milk and bring to a boil, then reduce heat to medium and cook, stirring consistently to prevent the flour from burning or overcooking, for about 3 – 4 minutes.
Reduce heat to low and add cheese in 3 batches, starting with American cheese. Whisk to fully combine, then follow with cheddar and Parmesan (whisk to fully combine after adding each cheese). Season with 1 teaspoon salt and 1/2 teaspoon pepper.
Fold the meat mixture into the cheese sauce, then fold in the pasta, and serve.