Heat olive oil in a large 10- or 12-inch cast iron skillet over medium-high heat. Add ground beef, season with Kosher salt, then cook, breaking up with a spatula, until evenly browned. Use a slotted spoon to transfer the beef to a bowl.
Add diced carrots, onion, celery, and rosemary sprigs, then season with the remaining Kosher salt and black pepper. Cook, stirring occasionally until the vegetables have browned slightly and become tender. Stir in tomato paste until dissolved.
Deglaze the pan. Allow this to reduce by about half and add Worcestershire sauce, Dijon mustard, and low-sodium beef broth. Reduce the heat to medium-low, then simmer.
Preheat an oven. Bring a large pot of salted water to a rapid boil. Add potatoes, then cook until tender. Add unsalted butter, milk, grated white Cheddar cheese, and Kosher salt, then stir until very well mixed. Taste, then adjust seasoning to taste.
Spoon the mashed potatoes on top – this can be a single smooth layer or you can create a decorative top. Transfer to a preheated oven and bake until the top is golden brown and bubbly. Enjoy immediately with fresh cracked pepper!