Place a 10” cast-iron skillet in oven and heat oven to 425°F.
While oven is heating, combine cornmeal, flour, sugar, baking powder and salt in a large bowl and set aside.
Whisk together mayonnaise and buttermilk, then mix into dry ingredients.
Stir in 1 cup of cheese and the diced jalapeños.
When oven is heated, carefully add shortening to pan, swirl to coat, then pour batter into pan. Top with remaining cheese and sliced jalapeños, if desired.
Bake for 30-40 minutes or until a toothpick inserted in the middle comes out clean.