Bring a 5 to 6-quart (5 to 6-litre) pot of salted water to a boil. Cook the fusilli 2 minutes less than the package directions. Reserve ½ cup (125 ml) pasta water, then drain and set aside.
Return the pot to the burner over medium heat. Form the ground beef into thin patties and place them in the dry pot. Press down to sear and let cook undisturbed until mostly browned. Break up the patties into bite-sized pieces, stir to finish cooking, then use a slotted spoon to transfer the beef to a bowl and set aside.
Reduce the heat to medium-low. Melt the butter in the same pot, leaving the beef fat for extra flavor if desired. Sprinkle in the flour and cook for 2 minutes without browning, stirring often.
Add the milk, yellow mustard, salt, and black pepper, stirring until smooth. Continue to cook for 3 to 5 minutes, or until the sauce has thickened slightly.
Add the grated cheddar cheese and cream cheese, stirring until melted and smooth. Stir in the diced tomatoes, chopped pickles, and reserved pasta water until fully combined.
Add the drained pasta, cooked ground beef, and diced onion to the sauce, stirring to coat everything evenly. Continue to cook for 5 minutes, or until everything is heated through. Serve hot.
Notes
Use a large, heavy-bottomed pot to prevent scorching. A wider surface area also helps the beef brown more evenly.
If the beef releases a lot of grease, you can spoon some out but leave a little behind for added flavor in the sauce.
Storage
Leftovers keep well in an airtight container in the fridge for up to 3 days. Reheat gently on the stovetop over low-medium heat, stirring in milk to loosen the sauce if needed. You can also gently reheat it in the microwave.
To freeze, transfer the cooled pasta to a large zipper-top bag, spreading it out into a flat, even layer. Remove as much air as possible before sealing to prevent freezer burn. To reheat, thaw it overnight in the fridge, then heat it in a pot on the stovetop with a splash of milk to loosen the sauce if needed.