01 - Bring a large pot of salted water to a boil. Add pasta shells or elbow macaroni and cook until al dente. Drain well and set aside.
02 - Heat oil in a large skillet over medium heat. Add diced onion and sauté until soft, about 3–4 minutes. Add ground beef and cook, breaking up meat with a wooden spoon, until browned. Season with salt and black pepper.
03 - Stir ketchup, yellow mustard, milk, cream cheese, and half of the shredded cheddar into the beef mixture. Continue cooking until cheese has melted and sauce is smooth.
04 - Add drained pasta to the skillet and toss gently to coat in the sauce. Sprinkle the remaining cheddar cheese over the top, cover, and allow the cheese to melt for 1 to 2 minutes.
05 - Remove skillet from heat. Top with chopped dill pickles, additional cheddar, and sesame seeds, if desired. Serve immediately.