1. Preheat the oven to 425°. Layer the sauerkraut, tomatoes, dressing, olives, and corned beef in a 10 1/2-inch cast iron skillet. Top with the cheese. Set aside.
2. Make the crust: Beat the egg in a medium bowl. Whisk in the buttermilk, milk, and oil; mix well. Add the corn meal mix, flour, and sugar; stir until smooth. Pour the batter evenly over the filling in the skillet. Bake until golden brown, 30 to 35 minutes.
3. Make the mustard sauce: Combine the mayonnaise, mustard, and onion in a small bowl; stir to blend.
4. Cut the casserole into wedges and serve with the mustard sauce.