1. Heat the oil in a 10″ or larger cast-iron skillet over medium-high heat.
2. Add the beef, onion, and spices. Brown the meat while stirring occasionally, about 6-7 minutes.
3. Stir in the tomato paste and half of the shredded cheese until well combined and the cheese is melty.
4. Top with dill pickle slices, red onion, tomatoes, and the remaining cheese.
5. Drizzle yellow mustard over the casserole and serve over a bed of shredded lettuce.
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